ABSTRACT The impact of mid-century climatic changes on crop productivity of winter wheat, maize, ... more ABSTRACT The impact of mid-century climatic changes on crop productivity of winter wheat, maize, potato and sugar beet was assessed for a temperate maritime climate in the Flemish Region, Belgium. Climatic projections of multiple regional and global climate models (RCMs from the EU-ENSEMBLES project and GCMs from the Coupled Model Intercomparison Project phase 3) were stochastically downscaled by the LARS-WG weather generator for use in the crop models AquaCrop and Sirius. Primarily positive effects on mean yield were simulated. Crops benefitted from elevated CO2., and from more radiation interception if the cropping period was adapted in response to higher temperatures. However, increased productivity was linked with increased susceptibility to water stress and greater inter-annual yield variability, particularly with adapted management. Impacts differed among and within ensembles of climate models, and among crops and environments. Although RCMs may be more suitable for local impact assessments than GCMs, inter-ensemble differences and contingent wider ranges of impacts with GCM projections found in this study indicate that applying RCMs driven by a limited number of GCMs alone would not give the full range of possible impacts. Further, this study suggests that the simulated inter-model variation can be larger than spatial variation within the region. These findings advocate the use of both GCM and RCM ensembles in assessments where temperature and precipitation are central, such as for crop production.
In order to adjust correctly the irrigationcalendar to actual weather and waterlimiting condition... more In order to adjust correctly the irrigationcalendar to actual weather and waterlimiting conditions, farmers requireguidelines to help them in their day to daydecisions. In this paper a procedure isoutlined to present such guidelines in theform of a single chart. On the chartinformation concerning the irrigationinterval and irrigation duration ispresented. Tables guide the user (i) toadjust the irrigation interval to theactual
Quinoa (Chenopodium quinoa Willd.) is a staple crop of the Altiplano of the high Andean region, u... more Quinoa (Chenopodium quinoa Willd.) is a staple crop of the Altiplano of the high Andean region, up to 4000m above sea level, which provides a highly nutritive food for the local and urban population, under adverse climatic conditions of frequent droughts, night frost and poor soils, and is regarded as having a great potential for export. With the aim of
Predicting attainable yield under water-limiting conditions is an important goal in arid, semi-ar... more Predicting attainable yield under water-limiting conditions is an important goal in arid, semi-arid and drought-prone environments. To address this task, FAO has developed a model, AquaCrop, which simulates attainable yields of the major herbaceous crops in response to water. Compared to other models, AquaCrop has a significantly smaller number of parameters and attempts to strike a balance between simplicity, accuracy,
Th e fi rst crop chosen to parameterize and test the new FAO AquaCrop model is maize (Zea mays L.... more Th e fi rst crop chosen to parameterize and test the new FAO AquaCrop model is maize (Zea mays L.). Working mainly with data sets from 6 yr of maize fi eld experiments at Davis, CA, plus another 4 yr of Davis maize canopy data, a set of conservative (nearly constant) parameters of AquaCrop, presumably applicable to widely diff erent conditions and not specifi c to a given crop cultivar, was evaluated by test simulations, and used to simulate the 6 yr of Davis data. Th e treatment variable was irrigation-withholding water aft er planting continuously, only up to tasseling, from tasseling onward, or intermittently, and with full irrigation (FI) as the control. From year to year, plant density (7-11.9 plants m −2 ), planting date (14 May-15 June), cultivar (a total of four), and atmospheric evaporative demand varied. Th e conservative parameters included: canopy growth and canopy decline coeffi cient (CDC); crop coeffi cient for transpiration (Tr) at full canopy; normalized water productivity for biomass (WP*); soil water depletion thresholds for the inhibition leaf growth and of stomatal conductance, and for the acceleration of canopy senescence; reference harvest index (HI o ); and coeffi cients for adjusting harvest index (HI) in relation to inhibition of leaf growth and of stomatal conductance. With all 19 parameters held constant, AquaCrop simulated the fi nal aboveground biomass within 10% of the measured value for at least 8 of the 13 treatments (6 yr of experiments) and also the grain yield for at least fi ve of the cases. In at least four of the cases, the simulated results were within 5% of the measured for biomass as well as for grain yield. Th e largest deviation between the simulated and measured values was 22% for biomass, and 24% for grain yield. Importantly, the simulated pattern of canopy progression and biomass accumulation over time were close to those measured, with Willmott's index of agreement (d) for 11 of the 13 cases being ≥0.98 for canopy cover (CC), and ≥0.97 for biomass. Accelerated senescence of canopy due to water stress, however, proved to be diffi cult to simulate accurately; of the six cases, the index of agreement for the worst one was 0.957 for canopy and 0.915 for biomass. Possible reasons for the discrepancies between the simulated and measured results include simplifi cations in the model and inaccuracies in measurements. Th e usefulness of AquaCrop with well-calibrated conservative parameters in assessing water use effi ciency (WUE) of a crops under diff erent conditions and in devising strategies to improve WUE is discussed. area index; p, fractional depletion of total available water in the root zone; PWP, permanent wilting point; SWC, soil water content; Tr, crop transpiration; WP, water productivity (for biomass); WP*, normalized (for evaporative demand and atmospheric CO 2 ) water productivity (for biomass); WUE, water use effi ciency.
Increasing irrigation efficiency in irrigation schemes has always been high on the agenda of poli... more Increasing irrigation efficiency in irrigation schemes has always been high on the agenda of policy makers. Despite quite some 'social' experiments, whereby large portions of management were carried over to the farmer level, results often remained disappointing. This paper explores why this came about for a case study in the Office du Niger irrigation scheme in Mali. Since Irrigation Management Transfer, farmers are responsible for the tertiary level, but collective action for water management remains below expectations. This paper applies an analytical framework based on the conceptual model developed by van Noordwijk et al. (Conserv Ecol 5:21, 2002) and proceeding in two steps. Firstly, an interview-based stakeholder analysis assesses the objectives and mental models of farmers and the central management regarding water management. It appears from the interviews that the central management wants to increase irrigation efficiency through fully-fledged collective action, whereas farmers value the latter only when it favours easy irrigation. Secondly, the relation between collective action and performance was tested through a field study on a sample of 36 tertiary blocks. Results indicate that only collective action at the inlet of the tertiary canal, currently implemented on less than a third of the tertiary blocks, increases irrigation efficiency (with 14%). Collective action for water allocation is implemented within almost three quarters of tertiary blocks and effectively reduces irrigation problems. However, if they lack the necessary social capital, not all farmers can establish collective action when needed. Based on this analysis, the paper proposes a mix of incentives and measures to resolve the conflict between farmers and the central management to their mutual benefit.
Journal of Irrigation and Drainage Engineering, 2005
Crop coefficient curves provide simple, reproducible means to estimate crop evapotranspiration (E... more Crop coefficient curves provide simple, reproducible means to estimate crop evapotranspiration (ET) from weather-based reference ET values. The dual crop coefficient ͑K c ͒ method of the Food and Agricultural Organization of the United States (FAO) Irrigation and Drainage Paper No. 56 (FAO-56) is intended to improve daily simulation of crop ET by considering separately the contribution of evaporation from soil. The dual method utilizes "basal" crop coefficients representing ET from crops having a dry soil surface and separately predicts evaporation from bare soil based on a water balance of the soil surface layer. Three extensions to the evaporation calculation procedure are described here that are intended to improve accuracy when applications warrant the extra complexity. The first extension uses parallel water balances representing the portion of the soil surface wetted by irrigation and precipitation together and the portion wetted by precipitation alone. The second extension uses three "stages" for surface drying and provides for application to deep cracking soils. The third extension predicts the extraction of the transpiration component from the soil surface layer. Sensitivity and analyses and illustrations indicate moderate sensitivity of daily calculated ET to application of the extensions. The dual K c procedure, although relatively simple computationally and structurally, estimates daily ET as measured by lysimeter relatively well for periods of bare soil and partial and full vegetation cover.
Journal of Irrigation and Drainage Engineering, 2005
The crop coefficient during the initial period ͑K c ini ͒ varies with wetting frequency, evaporat... more The crop coefficient during the initial period ͑K c ini ͒ varies with wetting frequency, evaporative demand, and water-holding capacity of the upper soil layer. It is possible to develop a semitheoretical integrated function to predict the average K c ini representing the initial period of a growing season when the soil is mostly bare and that incorporates these three factors. The function is based on a two-stage evaporation function as used in the Food and Agriculture Organization Irrigation and Drainage Paper No. 56 (FAO-56) dual crop coefficient method. Parameters in the integrated equation are soil based and can be calculated a priori without field measurements. The procedure can be used to produce graphical figures similar to that introduced in FAO-24 for K c ini . Similar to FAO-24, the function utilizes the mean time between wetting events and reference evapotranspiration. In this paper, the development of the procedure and figures for K c ini are described. Comparisons with measured evaporation and K c ini in southern California indicate relatively good performance by the function without calibration.
Daily runoff depths from 28 plots (5 m · 2 m) recorded during a 2-year period in the semi-arid to... more Daily runoff depths from 28 plots (5 m · 2 m) recorded during a 2-year period in the semi-arid to subhumid highlands of Tigray were analyzed to study the effect of vegetation restoration in exclosures and to identify other factors influencing runoff production. Plots are distributed over three study sites and located in different land use types and on different combinations of soil type, vegetation cover and slope gradient. Runoff was found to be significantly reduced when a degraded area is allowed to rehabilitate after closure. Runoff depth is significantly correlated with event variables such as rain depth, rainfall intensity, storm duration and soil moisture content. Total vegetation cover is the most important plot variable explaining about 80% of the variation in runoff coefficients through an exponential decay function. Also the runoff generating rainfall threshold has a positive correlation with total vegetation cover. Runoff was found to be negligible when the vegetation cover exceeds 65%. Other important variables affecting runoff production in the study sites are soil organic matter, soil bulk density, litter cover and slope gradient. a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j h y d r o l 224 K. Descheemaeker et al.
Although Jatropha curcas, an important tropical biofuel crop, is reputed for its drought resistan... more Although Jatropha curcas, an important tropical biofuel crop, is reputed for its drought resistance, its ability to perform under dry conditions has hardly been investigated. In a greenhouse experiment we investigated the plant-water relationships of Jatropha seedlings from different accessions under different levels of drought stress. There was little difference in plant-water relations between accessions. Drought significantly reduced leaf area, biomass and relative growth rate, but had no effect on specific leaf area, daily range in leaf water potential, leaf water content, transpiration efficiency or aboveground biomass water productivity, corrected for atmospheric conditions. Stem wood density was equally low (0.26 g cm À3 ) for all treatments. Stem water content was lowest for dry treatment seedlings. Based on these results, Jatropha could be characterized as a stem-succulent tree. In contrast to other stem-succulent deciduous trees, leaves were not shed immediately after the seedlings were confronted with drought. Instead, at the onset of drought, leaves with a higher adaxial stomatal density were formed, after which leaves were only gradually shed. The role of the succulent stem in the water economy of Jatropha was confined to balancing the small water losses of the leaves during drought.
The agroclimatic potential was analyzed for four locations in the Bolivian Altiplano (highland) a... more The agroclimatic potential was analyzed for four locations in the Bolivian Altiplano (highland) and used to delineate regional agroclimatic variability. A downward trend from North to South was observed in temperature, the onset and the duration of the rainy season, indicating that climate-wise crop production is only possible during 5 months in the north and less than 4 months in the south. A clear negative relationship between the day of onset and the duration of the rainy season was detected. Even in years with 50% dependable precipitation, the cumulative rainfall depth is less than the reference evapotranspiration calculated by the Penman-Monteith equation, and the erratic occurrence of long dry spells seriously affect crop growth. The adverse climatic conditions call for an agroclimatic-based action plan to improve the agricultural output of a region blessed with high and fairly constant levels of radiation. Actions worth to be considered are the introduction of deficit irrigation, protected intensive cropping, and the cultivation of crop species and varieties that require a short growing season, tolerate low water availability, and are resistant to early or late frosts. r
The paper estimates the water requirements for salt control in rice schemes located on saline sol... more The paper estimates the water requirements for salt control in rice schemes located on saline solls in the Senegal river delta. When the fields are not culfivated, salts a.re transported to the top soff by capillary rise from the very saline and shallow ground water table. During the irrigation season, the large quanfity of irfigation water adds addifional salt to the fields. If the percolation rate of the soff is small, salts will have to be removed from the schemes by flushing the standing water from the fields when the salinity of the watet reaches the threshold value of 1.5 ds m -l. The results indicate that if the schemes are located on the river banks (fondé), flushing at the beginning of the season and the percolation losses throughout the season may be sufficient to keep the salts out of the root zone. On the less permeable solls in the depressions (hollaldé), an extra flushing is required to evacuate enough salts from the fields during the irrigation season. In total about 2,300 m a ha -1 of water may be needed for flushing. If flushing is not pracfised, the schelnes have to be abandoned after a few years of culfivafion due to build-up of soff salinity.
The water use of rice schemes was derived from field measurements made during the 1991 and 1992 s... more The water use of rice schemes was derived from field measurements made during the 1991 and 1992 season at 13 irrigation schemes situated in the delta and valley of the Senegal river. The seasonal watet consumption, as well as the water distribution throughout the season and the operation of the pumping stations were evaluated. The watet consumption was compared with standards which ranged from 16,650 to 21,100 m 3 ha -1 depending on the season and the location of the scheme. Half of the analyzed schemes used more watet than required. Analysis of the water distribution throughout the season revealed that in general the supply follows the demand. In small-scale schemes however, the fit between supply and demand is often lacking, resulting in crop stress and yield reduction. Furthermore, the study shows that the capacity of half of the pumping stations are under-used as a result of a long stretched saturation period and/or the irrigation of only a small fraction of the total area of the scheme.
The Office du Niger is a centrally managed collective irrigation scheme of 80 000 ha, mainly cult... more The Office du Niger is a centrally managed collective irrigation scheme of 80 000 ha, mainly cultivated with flooded rice. In the context of recent reforms, water distribution and maintenance at the tertiary canal level were left to farmers. In this paper, their ability to resolve collective action problems through devising, monitoring and enforcing rules is diagnosed through a questionnaire survey of 89 farmers on 59 tertiary canals from five villages. Results show that rules are devised only on 30 and 24% of the canals for water distribution and maintenance respectively. Moreover, there is often no consensus on rules among farmers, and monitoring and sanctioning mechanisms are absent. This results in individualistic behavior causing problems concerning water distribution and maintenance for respectively 20 and 43% of the interviewed farmers. Ineffectiveness of peer pressure and farmers' incomplete mentality shift towards assuming collective responsibility are impediments to successful organization of water management. With water supply being abundant and the infrastructure recently rehabilitated, organization of water management at tertiary level is, however, not always required in order to avoid problems. On the other hand, the current state of affairs is not considered sustainable, as the irrigated area will strongly expand and the irrigation infrastructure ages with time. Measures of sensitization and group empowerment accompanying the process of management transfer will therefore be desirable.
... Singh et al. (2001) proposed a linear programming model for the optimal cropping pattern in a... more ... Singh et al. (2001) proposed a linear programming model for the optimal cropping pattern in a canal command area. Kumar et al. ... Kuo et al. (2000) proposed a genetic algorithm optimisation model for the optimal cropping pattern in an irrigation scheme. ...
Hydrological modelling often implies the use of rainfall data. Its quality and resolution directl... more Hydrological modelling often implies the use of rainfall data. Its quality and resolution directly affect the accuracy of the simulation results. This study illustrates that a simple approach of incorporating rainfall intensity information in daily rainfall records significantly improves the simulation of surface runoff and rainfall infiltration into soil profiles. The procedure is developed using a frequency analysis on rainfall data of the Royal Meteorological Institute of Belgium, collected with a resolution of 10 min and for a consecutive period of 61 years. The frequency analysis of the data allowed the incorporation of rainfall intensity information into daily rainfall records. To test the effect of this approach the surface runoff and water flow into three different soil types was simulated using the HYDRUS-1D model for a typical dry, normal and wet year. The simulation results whereby the observed 10-min rainfall data was used as input were considered as the reference. Comparative analysis revealed that the simulations using the 10 min rainfall data deducted from the incorporation of rainfall intensity into daily rainfall records, deviate a maximum 1Ð2% from the reference and produce much better results than the Soil Conservation Service (SCS) runoff curve-number method because rainfall intensity is considered in the procedure presented. The SCS curve-number method typical overestimates surface runoff during periods of low rainfall intensity (winter) and underestimate runoff during periods of high rainfall intensities (summer).
An optimization model has been developed toaid decision making in real time fordeficit irrigation... more An optimization model has been developed toaid decision making in real time fordeficit irrigation when conflict betweenwater supply and demand arises in amultiple crop irrigation scheme. The resultis the optimal allocation of short-termsupply of irrigation water. Theoptimization model is based on DynamicProgramming. In the optimization model, theshort-term supply is optimized in functionof a specified strategy determined by theuser. The strategies
In the Tigray highlands of Northern Ethiopia, the establishment of exclosures (i.e., areas closed... more In the Tigray highlands of Northern Ethiopia, the establishment of exclosures (i.e., areas closed for grazing and agriculture) has become an important measure to combat land degradation and restore vegetative cover. Exclosures are commonly found on steep slopes and downslope from a sediment source area. In this study their sediment trapping capacity and controlling factors were investigated. Total sediment depth turned out to be related to vegetation cover, sediment source area and in some cases slope gradient. Thickness of recent, short-term (b 20 years) sediment deposits was strongly related to distance from the top edge of the closed area, slope gradient, vegetation cover and characteristics of the sediment source area. Mean sediment deposition rates ranged between 26 and 123 Mg ha À 1 yr À 1 . Under influence of vegetation and sediment deposition dark soils rich in organic matter (Phaeozems) develop. In view of their high sediment trapping capacity, exclosures are highly valued as efficient soil conservation measures in the Tigray highlands. D
ABSTRACT The impact of mid-century climatic changes on crop productivity of winter wheat, maize, ... more ABSTRACT The impact of mid-century climatic changes on crop productivity of winter wheat, maize, potato and sugar beet was assessed for a temperate maritime climate in the Flemish Region, Belgium. Climatic projections of multiple regional and global climate models (RCMs from the EU-ENSEMBLES project and GCMs from the Coupled Model Intercomparison Project phase 3) were stochastically downscaled by the LARS-WG weather generator for use in the crop models AquaCrop and Sirius. Primarily positive effects on mean yield were simulated. Crops benefitted from elevated CO2., and from more radiation interception if the cropping period was adapted in response to higher temperatures. However, increased productivity was linked with increased susceptibility to water stress and greater inter-annual yield variability, particularly with adapted management. Impacts differed among and within ensembles of climate models, and among crops and environments. Although RCMs may be more suitable for local impact assessments than GCMs, inter-ensemble differences and contingent wider ranges of impacts with GCM projections found in this study indicate that applying RCMs driven by a limited number of GCMs alone would not give the full range of possible impacts. Further, this study suggests that the simulated inter-model variation can be larger than spatial variation within the region. These findings advocate the use of both GCM and RCM ensembles in assessments where temperature and precipitation are central, such as for crop production.
In order to adjust correctly the irrigationcalendar to actual weather and waterlimiting condition... more In order to adjust correctly the irrigationcalendar to actual weather and waterlimiting conditions, farmers requireguidelines to help them in their day to daydecisions. In this paper a procedure isoutlined to present such guidelines in theform of a single chart. On the chartinformation concerning the irrigationinterval and irrigation duration ispresented. Tables guide the user (i) toadjust the irrigation interval to theactual
Quinoa (Chenopodium quinoa Willd.) is a staple crop of the Altiplano of the high Andean region, u... more Quinoa (Chenopodium quinoa Willd.) is a staple crop of the Altiplano of the high Andean region, up to 4000m above sea level, which provides a highly nutritive food for the local and urban population, under adverse climatic conditions of frequent droughts, night frost and poor soils, and is regarded as having a great potential for export. With the aim of
Predicting attainable yield under water-limiting conditions is an important goal in arid, semi-ar... more Predicting attainable yield under water-limiting conditions is an important goal in arid, semi-arid and drought-prone environments. To address this task, FAO has developed a model, AquaCrop, which simulates attainable yields of the major herbaceous crops in response to water. Compared to other models, AquaCrop has a significantly smaller number of parameters and attempts to strike a balance between simplicity, accuracy,
Th e fi rst crop chosen to parameterize and test the new FAO AquaCrop model is maize (Zea mays L.... more Th e fi rst crop chosen to parameterize and test the new FAO AquaCrop model is maize (Zea mays L.). Working mainly with data sets from 6 yr of maize fi eld experiments at Davis, CA, plus another 4 yr of Davis maize canopy data, a set of conservative (nearly constant) parameters of AquaCrop, presumably applicable to widely diff erent conditions and not specifi c to a given crop cultivar, was evaluated by test simulations, and used to simulate the 6 yr of Davis data. Th e treatment variable was irrigation-withholding water aft er planting continuously, only up to tasseling, from tasseling onward, or intermittently, and with full irrigation (FI) as the control. From year to year, plant density (7-11.9 plants m −2 ), planting date (14 May-15 June), cultivar (a total of four), and atmospheric evaporative demand varied. Th e conservative parameters included: canopy growth and canopy decline coeffi cient (CDC); crop coeffi cient for transpiration (Tr) at full canopy; normalized water productivity for biomass (WP*); soil water depletion thresholds for the inhibition leaf growth and of stomatal conductance, and for the acceleration of canopy senescence; reference harvest index (HI o ); and coeffi cients for adjusting harvest index (HI) in relation to inhibition of leaf growth and of stomatal conductance. With all 19 parameters held constant, AquaCrop simulated the fi nal aboveground biomass within 10% of the measured value for at least 8 of the 13 treatments (6 yr of experiments) and also the grain yield for at least fi ve of the cases. In at least four of the cases, the simulated results were within 5% of the measured for biomass as well as for grain yield. Th e largest deviation between the simulated and measured values was 22% for biomass, and 24% for grain yield. Importantly, the simulated pattern of canopy progression and biomass accumulation over time were close to those measured, with Willmott's index of agreement (d) for 11 of the 13 cases being ≥0.98 for canopy cover (CC), and ≥0.97 for biomass. Accelerated senescence of canopy due to water stress, however, proved to be diffi cult to simulate accurately; of the six cases, the index of agreement for the worst one was 0.957 for canopy and 0.915 for biomass. Possible reasons for the discrepancies between the simulated and measured results include simplifi cations in the model and inaccuracies in measurements. Th e usefulness of AquaCrop with well-calibrated conservative parameters in assessing water use effi ciency (WUE) of a crops under diff erent conditions and in devising strategies to improve WUE is discussed. area index; p, fractional depletion of total available water in the root zone; PWP, permanent wilting point; SWC, soil water content; Tr, crop transpiration; WP, water productivity (for biomass); WP*, normalized (for evaporative demand and atmospheric CO 2 ) water productivity (for biomass); WUE, water use effi ciency.
Increasing irrigation efficiency in irrigation schemes has always been high on the agenda of poli... more Increasing irrigation efficiency in irrigation schemes has always been high on the agenda of policy makers. Despite quite some 'social' experiments, whereby large portions of management were carried over to the farmer level, results often remained disappointing. This paper explores why this came about for a case study in the Office du Niger irrigation scheme in Mali. Since Irrigation Management Transfer, farmers are responsible for the tertiary level, but collective action for water management remains below expectations. This paper applies an analytical framework based on the conceptual model developed by van Noordwijk et al. (Conserv Ecol 5:21, 2002) and proceeding in two steps. Firstly, an interview-based stakeholder analysis assesses the objectives and mental models of farmers and the central management regarding water management. It appears from the interviews that the central management wants to increase irrigation efficiency through fully-fledged collective action, whereas farmers value the latter only when it favours easy irrigation. Secondly, the relation between collective action and performance was tested through a field study on a sample of 36 tertiary blocks. Results indicate that only collective action at the inlet of the tertiary canal, currently implemented on less than a third of the tertiary blocks, increases irrigation efficiency (with 14%). Collective action for water allocation is implemented within almost three quarters of tertiary blocks and effectively reduces irrigation problems. However, if they lack the necessary social capital, not all farmers can establish collective action when needed. Based on this analysis, the paper proposes a mix of incentives and measures to resolve the conflict between farmers and the central management to their mutual benefit.
Journal of Irrigation and Drainage Engineering, 2005
Crop coefficient curves provide simple, reproducible means to estimate crop evapotranspiration (E... more Crop coefficient curves provide simple, reproducible means to estimate crop evapotranspiration (ET) from weather-based reference ET values. The dual crop coefficient ͑K c ͒ method of the Food and Agricultural Organization of the United States (FAO) Irrigation and Drainage Paper No. 56 (FAO-56) is intended to improve daily simulation of crop ET by considering separately the contribution of evaporation from soil. The dual method utilizes "basal" crop coefficients representing ET from crops having a dry soil surface and separately predicts evaporation from bare soil based on a water balance of the soil surface layer. Three extensions to the evaporation calculation procedure are described here that are intended to improve accuracy when applications warrant the extra complexity. The first extension uses parallel water balances representing the portion of the soil surface wetted by irrigation and precipitation together and the portion wetted by precipitation alone. The second extension uses three "stages" for surface drying and provides for application to deep cracking soils. The third extension predicts the extraction of the transpiration component from the soil surface layer. Sensitivity and analyses and illustrations indicate moderate sensitivity of daily calculated ET to application of the extensions. The dual K c procedure, although relatively simple computationally and structurally, estimates daily ET as measured by lysimeter relatively well for periods of bare soil and partial and full vegetation cover.
Journal of Irrigation and Drainage Engineering, 2005
The crop coefficient during the initial period ͑K c ini ͒ varies with wetting frequency, evaporat... more The crop coefficient during the initial period ͑K c ini ͒ varies with wetting frequency, evaporative demand, and water-holding capacity of the upper soil layer. It is possible to develop a semitheoretical integrated function to predict the average K c ini representing the initial period of a growing season when the soil is mostly bare and that incorporates these three factors. The function is based on a two-stage evaporation function as used in the Food and Agriculture Organization Irrigation and Drainage Paper No. 56 (FAO-56) dual crop coefficient method. Parameters in the integrated equation are soil based and can be calculated a priori without field measurements. The procedure can be used to produce graphical figures similar to that introduced in FAO-24 for K c ini . Similar to FAO-24, the function utilizes the mean time between wetting events and reference evapotranspiration. In this paper, the development of the procedure and figures for K c ini are described. Comparisons with measured evaporation and K c ini in southern California indicate relatively good performance by the function without calibration.
Daily runoff depths from 28 plots (5 m · 2 m) recorded during a 2-year period in the semi-arid to... more Daily runoff depths from 28 plots (5 m · 2 m) recorded during a 2-year period in the semi-arid to subhumid highlands of Tigray were analyzed to study the effect of vegetation restoration in exclosures and to identify other factors influencing runoff production. Plots are distributed over three study sites and located in different land use types and on different combinations of soil type, vegetation cover and slope gradient. Runoff was found to be significantly reduced when a degraded area is allowed to rehabilitate after closure. Runoff depth is significantly correlated with event variables such as rain depth, rainfall intensity, storm duration and soil moisture content. Total vegetation cover is the most important plot variable explaining about 80% of the variation in runoff coefficients through an exponential decay function. Also the runoff generating rainfall threshold has a positive correlation with total vegetation cover. Runoff was found to be negligible when the vegetation cover exceeds 65%. Other important variables affecting runoff production in the study sites are soil organic matter, soil bulk density, litter cover and slope gradient. a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j h y d r o l 224 K. Descheemaeker et al.
Although Jatropha curcas, an important tropical biofuel crop, is reputed for its drought resistan... more Although Jatropha curcas, an important tropical biofuel crop, is reputed for its drought resistance, its ability to perform under dry conditions has hardly been investigated. In a greenhouse experiment we investigated the plant-water relationships of Jatropha seedlings from different accessions under different levels of drought stress. There was little difference in plant-water relations between accessions. Drought significantly reduced leaf area, biomass and relative growth rate, but had no effect on specific leaf area, daily range in leaf water potential, leaf water content, transpiration efficiency or aboveground biomass water productivity, corrected for atmospheric conditions. Stem wood density was equally low (0.26 g cm À3 ) for all treatments. Stem water content was lowest for dry treatment seedlings. Based on these results, Jatropha could be characterized as a stem-succulent tree. In contrast to other stem-succulent deciduous trees, leaves were not shed immediately after the seedlings were confronted with drought. Instead, at the onset of drought, leaves with a higher adaxial stomatal density were formed, after which leaves were only gradually shed. The role of the succulent stem in the water economy of Jatropha was confined to balancing the small water losses of the leaves during drought.
The agroclimatic potential was analyzed for four locations in the Bolivian Altiplano (highland) a... more The agroclimatic potential was analyzed for four locations in the Bolivian Altiplano (highland) and used to delineate regional agroclimatic variability. A downward trend from North to South was observed in temperature, the onset and the duration of the rainy season, indicating that climate-wise crop production is only possible during 5 months in the north and less than 4 months in the south. A clear negative relationship between the day of onset and the duration of the rainy season was detected. Even in years with 50% dependable precipitation, the cumulative rainfall depth is less than the reference evapotranspiration calculated by the Penman-Monteith equation, and the erratic occurrence of long dry spells seriously affect crop growth. The adverse climatic conditions call for an agroclimatic-based action plan to improve the agricultural output of a region blessed with high and fairly constant levels of radiation. Actions worth to be considered are the introduction of deficit irrigation, protected intensive cropping, and the cultivation of crop species and varieties that require a short growing season, tolerate low water availability, and are resistant to early or late frosts. r
The paper estimates the water requirements for salt control in rice schemes located on saline sol... more The paper estimates the water requirements for salt control in rice schemes located on saline solls in the Senegal river delta. When the fields are not culfivated, salts a.re transported to the top soff by capillary rise from the very saline and shallow ground water table. During the irrigation season, the large quanfity of irfigation water adds addifional salt to the fields. If the percolation rate of the soff is small, salts will have to be removed from the schemes by flushing the standing water from the fields when the salinity of the watet reaches the threshold value of 1.5 ds m -l. The results indicate that if the schemes are located on the river banks (fondé), flushing at the beginning of the season and the percolation losses throughout the season may be sufficient to keep the salts out of the root zone. On the less permeable solls in the depressions (hollaldé), an extra flushing is required to evacuate enough salts from the fields during the irrigation season. In total about 2,300 m a ha -1 of water may be needed for flushing. If flushing is not pracfised, the schelnes have to be abandoned after a few years of culfivafion due to build-up of soff salinity.
The water use of rice schemes was derived from field measurements made during the 1991 and 1992 s... more The water use of rice schemes was derived from field measurements made during the 1991 and 1992 season at 13 irrigation schemes situated in the delta and valley of the Senegal river. The seasonal watet consumption, as well as the water distribution throughout the season and the operation of the pumping stations were evaluated. The watet consumption was compared with standards which ranged from 16,650 to 21,100 m 3 ha -1 depending on the season and the location of the scheme. Half of the analyzed schemes used more watet than required. Analysis of the water distribution throughout the season revealed that in general the supply follows the demand. In small-scale schemes however, the fit between supply and demand is often lacking, resulting in crop stress and yield reduction. Furthermore, the study shows that the capacity of half of the pumping stations are under-used as a result of a long stretched saturation period and/or the irrigation of only a small fraction of the total area of the scheme.
The Office du Niger is a centrally managed collective irrigation scheme of 80 000 ha, mainly cult... more The Office du Niger is a centrally managed collective irrigation scheme of 80 000 ha, mainly cultivated with flooded rice. In the context of recent reforms, water distribution and maintenance at the tertiary canal level were left to farmers. In this paper, their ability to resolve collective action problems through devising, monitoring and enforcing rules is diagnosed through a questionnaire survey of 89 farmers on 59 tertiary canals from five villages. Results show that rules are devised only on 30 and 24% of the canals for water distribution and maintenance respectively. Moreover, there is often no consensus on rules among farmers, and monitoring and sanctioning mechanisms are absent. This results in individualistic behavior causing problems concerning water distribution and maintenance for respectively 20 and 43% of the interviewed farmers. Ineffectiveness of peer pressure and farmers' incomplete mentality shift towards assuming collective responsibility are impediments to successful organization of water management. With water supply being abundant and the infrastructure recently rehabilitated, organization of water management at tertiary level is, however, not always required in order to avoid problems. On the other hand, the current state of affairs is not considered sustainable, as the irrigated area will strongly expand and the irrigation infrastructure ages with time. Measures of sensitization and group empowerment accompanying the process of management transfer will therefore be desirable.
... Singh et al. (2001) proposed a linear programming model for the optimal cropping pattern in a... more ... Singh et al. (2001) proposed a linear programming model for the optimal cropping pattern in a canal command area. Kumar et al. ... Kuo et al. (2000) proposed a genetic algorithm optimisation model for the optimal cropping pattern in an irrigation scheme. ...
Hydrological modelling often implies the use of rainfall data. Its quality and resolution directl... more Hydrological modelling often implies the use of rainfall data. Its quality and resolution directly affect the accuracy of the simulation results. This study illustrates that a simple approach of incorporating rainfall intensity information in daily rainfall records significantly improves the simulation of surface runoff and rainfall infiltration into soil profiles. The procedure is developed using a frequency analysis on rainfall data of the Royal Meteorological Institute of Belgium, collected with a resolution of 10 min and for a consecutive period of 61 years. The frequency analysis of the data allowed the incorporation of rainfall intensity information into daily rainfall records. To test the effect of this approach the surface runoff and water flow into three different soil types was simulated using the HYDRUS-1D model for a typical dry, normal and wet year. The simulation results whereby the observed 10-min rainfall data was used as input were considered as the reference. Comparative analysis revealed that the simulations using the 10 min rainfall data deducted from the incorporation of rainfall intensity into daily rainfall records, deviate a maximum 1Ð2% from the reference and produce much better results than the Soil Conservation Service (SCS) runoff curve-number method because rainfall intensity is considered in the procedure presented. The SCS curve-number method typical overestimates surface runoff during periods of low rainfall intensity (winter) and underestimate runoff during periods of high rainfall intensities (summer).
An optimization model has been developed toaid decision making in real time fordeficit irrigation... more An optimization model has been developed toaid decision making in real time fordeficit irrigation when conflict betweenwater supply and demand arises in amultiple crop irrigation scheme. The resultis the optimal allocation of short-termsupply of irrigation water. Theoptimization model is based on DynamicProgramming. In the optimization model, theshort-term supply is optimized in functionof a specified strategy determined by theuser. The strategies
In the Tigray highlands of Northern Ethiopia, the establishment of exclosures (i.e., areas closed... more In the Tigray highlands of Northern Ethiopia, the establishment of exclosures (i.e., areas closed for grazing and agriculture) has become an important measure to combat land degradation and restore vegetative cover. Exclosures are commonly found on steep slopes and downslope from a sediment source area. In this study their sediment trapping capacity and controlling factors were investigated. Total sediment depth turned out to be related to vegetation cover, sediment source area and in some cases slope gradient. Thickness of recent, short-term (b 20 years) sediment deposits was strongly related to distance from the top edge of the closed area, slope gradient, vegetation cover and characteristics of the sediment source area. Mean sediment deposition rates ranged between 26 and 123 Mg ha À 1 yr À 1 . Under influence of vegetation and sediment deposition dark soils rich in organic matter (Phaeozems) develop. In view of their high sediment trapping capacity, exclosures are highly valued as efficient soil conservation measures in the Tigray highlands. D
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