Sustainable Management

The development of distributed modelling technology has been hampered by incomplete understanding of spatially-variable processes and the lack of adequate data to characterize spatially varying inputs and state variables. This seems to be particularly true with respect to rainfall fields. This paper examines the effect of uncertainty in spatial estimates of rainfall on predictions of runoff volume, peak runoff and sediment yield. This is achieved by applying a distributed runoff-erosion model to a 6.73 km' catchment with a dense recording raingauge network in an area dominated by convective air-mass storm rainfall and infiltration-excess overland flow. Results indicate that the density of the raingauge network, the spatial arrangement of individual raingauges within the catchment, and the spatio-temporal characteristics of storm events all greatly influence catchment response predictions. The implication of this is that first, the utility of a spatially distributed approach to model catchment response is compromised by large uncertainties in the spatial distribution of rainfall input. Second, application of distributed catchment models may require knowledge of the spatial variability of rainfall at the same scale used to represent the catchment area and geometry. Third, estimates of areal rainfall using traditional techniques like the Thiessen method may be accurate at the catchment scale but they do not produce accurate estimates of rainfall for distributed catchment modelling. Kepvords: distributed modelling: uncertainty: areal rainfall; overland flow; soil erosion: sediment yield: dryland hydrology ' Present address:

In recent years, distributed (spatially explicit) models have become more widely used, in part because of the availability of geographic information systems. However, advances in the understanding of hydrologic and geomorphic processes in concert with more readily available spatial data sets make these models more useful. In particular, distributed catchment models have the potential to predict where and when runoff is generated and sediment entrainment or deposition is occurring. To ensure that the models have the potential to produce physically realistic sediment yield predictions, it is necessary to identify unique sediment contributions from raindrop impact and entrainment by flowing water; which are highly scale-dependent processes. In this study, we used four different representations of a 4.4 ha experimental catchment with the most complex representation preserving the channel network detail as observed in the field. Our objective was to determine whether it is possible to id...

Integrated Watershed Management (IWM) has emerged worldwide as the preferred model for watershed planning. IWM uses the watershed as the basic geographic planning unit while integrating social, economic, ecological and policy concerns with science to develop the best plan. ...

Distributed process-based hydrologic models have been used to describe and predict the movement of sediment on small watersheds. However, to parameterize these models requires an understanding of the spatial variability of erosion processes and the particle sizes of the sediment Ž. being moved. In this study, a high resolution digital elevation model DEM and detailed sediment particle sampling allowed a comparison of hillslope characteristics and particle sizes of surficial armoring in a semiarid watershed. Individual particle size classes on hillslopes are correlated with the underlying sediment type, local slope, aspect, and area draining through a grid element. The strongest correlations are between the underlying sediment and overlying sediment. However, the distribution of the particle size classes is consistent with a hydrodynamic explanation for sorting. In particular, increased area draining through a grid node and increased slope are correlated with higher concentrations of the 16-64-mm particle size class. Both the coarsest and finest particle size classes are significantly correlated with the aspect of flow from a grid cell, with increased coarse particles and decreased fines on east-facing slopes. These spatial differences with aspect are attributed to dry season prevailing winds. These observations about process and spatial distribution are useful in predicting the spatial distribution of particles on the watershed for applications such as distributed hydrologic models.

The general problem of solving for normal flow dcplh in open-chan nel flow has a complication in that some types of channel cross sections do not always have a unique solution. This paper analyzes an alternative iterative pro cedure for quickly and accurately solving the implicit problem of determining the normal flow depth in complex channel sections. Conditions that guarantee a unique solution and guarantee that the iterative procedure will converge to the solution are developed. A computer program for quickly and accurately finding the unique solution, using the Chezy or Manning flow resistance equations, is available. Test runs for a rectangular, a triangular, a trapezoidal, and two complex channel cross sections are used to evaluate the effectiveness of the procedure. The test results show that the iterative procedure presented here meets the requirements of guar anteed convergence, computational efficiency (speed and accuracy), and the ability to handle both trapezoidal and complex channel cross sections.

Distributed process-based hydrologic models have been used to describe and predict the movement of sediment on small watersheds. However, to parameterize these models requires an understanding of the spatial variability of erosion processes and the particle sizes of the sediment Ž. being moved. In this study, a high resolution digital elevation model DEM and detailed sediment particle sampling allowed a comparison of hillslope characteristics and particle sizes of surficial armoring in a semiarid watershed. Individual particle size classes on hillslopes are correlated with the underlying sediment type, local slope, aspect, and area draining through a grid element. The strongest correlations are between the underlying sediment and overlying sediment. However, the distribution of the particle size classes is consistent with a hydrodynamic explanation for sorting. In particular, increased area draining through a grid node and increased slope are correlated with higher concentrations of the 16-64-mm particle size class. Both the coarsest and finest particle size classes are significantly correlated with the aspect of flow from a grid cell, with increased coarse particles and decreased fines on east-facing slopes. These spatial differences with aspect are attributed to dry season prevailing winds. These observations about process and spatial distribution are useful in predicting the spatial distribution of particles on the watershed for applications such as distributed hydrologic models.

Hydraulic, sediment, land-use, and rock-erosivity data of 22 alluvial streams were used to evaluate conditions of bedload transport and the performance of selected bedload-transport equations. Transport categories of transport-limited ͑TL͒, partially transportlimited ͑PTL͒, and supply-limited ͑SL͒ were identified by a semiquantitative approach that considers hydraulic constraints on sediment movement and the processes that control sediment availability at the basin scale. Equations by Parker et al. in 1982, Schoklitsch in 1962, and Meyer-Peter and Muller in 1948 adequately predicted sediment transport in channels with TL condition, whereas the equations of Bagnold in 1980, and Schoklitsch, in 1962, performed well for PTL and SL conditions. Overall, the equation of Schoklitsch predicted well the measured bedload data for eight of 22 streams, and the Bagnold equation predicted the measured data in seven streams.

Sediment represents a major non-point source pollutant throughout the world. In addition to reduced agricultural productivity as the result of the loss of fertile soil, soil erosion also can have significant water-quality impacts in downstream waterbodies, reducing water transparency, degrading aquatic habitats and reducing the operational life and water storage capacity of reservoirs producing hydroelectric power. Various other pollutants also can absorb to sediment particles, creating additional downstream water-quality concerns for humans and the natural environment. In view of its human and environmental significance, two indices (an erosion index and runoff index) were developed to identify areas within the US portion of the Rio Grande Basin exhibiting physical characteristics conducive to producing significant non-point source pollution loads, focusing on land erosion as a sediment source. The erosion index is an adaptation of the Universal Soil Loss Equation, being the product of rainfall erosivity (R factor), soil erodibility (K factor), and a topographic factor (LS factor). The erosion index correlated well with measurements of sediment yields from runoff plots. The Curve Number was used as the runoff index. In conjunction with identified pollutant-generating land uses, or source landscapes, these indices were used to identify sub-watersheds within the US portion of the Rio Grande Basin that merit further investigation for non-point source pollution prevention and control via the use of hydrologic modelling techniques.

The Simplex and the Shuffled Complex Evolution-University of Arizona (SCE-UA) optimization algorithms and the simple least-squares (SLS) and the heteroscedastic maximum likelihood estimator (HMLE) were applied to identify erosion parameters for a process-based mode1 of catchment runoff and sediment yield. Four parameter identification problems were posed with the objective of studying the behaviour of the model under four optimization procedures. Error-free data as well as data with correlated error provided the means for determining the effectiveness of the optimization procedures studied. Results showed that the Simplex algorithm and the SLS objective function provided the best parameter estimates at lower levels of error, while the Simplex algorithm and HMLE objective function provided the best parameter estimates at higher levels of error. This was contrary to the expected theoretical outcome since the SCE-UA algorithm was more successful than the simplex algorithm in locating the global minimum on the response surface.

A process-based erosion model is used to study parameterization problems of sediment entrainment equations in overland flow areas. One of the equations for entrainment by flow is developed based on a theory of excess stream power, while the other two relate to excess hydraulic shear. The investigation is conducted in two steps. The first step examines parameter optimization for simulated data sets where the parameter values are known. In the second step, parameter optimization for the most robust equation is examined using experimental data from rainfall simulator plots. Results demonstrate that although the model is capable of estimating total sediment yields with relatively small errors in parameter estimates, the converse is true when the optimization is performed for sediment concentrations. Although sediment yields calculated from simulated sediment concentrations match well with observed data, the parameter estimates generally underestimate sediment concentrations on the rising limb of the sediment graphs, and they overestimate them on the falling limb. This difficulty might be related to structural problems in the model, and unique solutions for parameter estimates cannot be obtained.

The question of watershed response to changes in land use and land cover has received a great deal of attention in the hydrologic literature. One of the primary tools for quantifying such changes has been the paired watershed approach. In general this approach applies a linear regression analysis to relate water yield from a control watershed to a physiographically similar watershed in close proximity, and to predict changes in yield following treatment. Although much research on paired watershed experiments has focused on quantification of the impacts of land-use and land cover changes on water yield, little attention has been brought to evaluating explicitly the methodology used to quantify such effects. An alternative method is proposed for examining treatment impacts and their duration, through the application of a cumulative recursive residual test of model stability. The results from a paired watershed study of ponderosa pine watersheds in north-central Arizona are analyzed using both a traditional linear regression analysis and a recursive residuals approach. The results suggest that the linear regression approach may fail to account for the true complexity of watershed response to vegetation treatments, by underestimating the interactions of treatment impacts, shifts in climatic drivers, and revegetation rates.

A semiquantitative analysis was used to assess the impacts of water rights appropriations and allocations in the Upper Rio Grande (URG) Basin, in Colorado. The study also explores the causes and effects of the changes on collateral elements in the URG agriculture system, namely the San Luis Valley (SLV) in Colorado. Population increases, after the acquisition of the territory from 1854 to 1900, were the major cause of increased acquisitions of surface water rights. By 1912 surface waters were nearly 100% appropriated. The population continued to increase until 1930 after which it remained stable. Water users began making large increases in the number of appropriations of groundwater around 1925, with the majority of increases starting around 1935. As a result, moratoriums were placed on well development in the 1970s and 80s. Individual spikes in water rights acquisitions of surface and groundwater were associated with periodic droughts and high crop prices. Change point time series analysis identified three major periods of acquisition of waters rights

The general problem of solving for normal flow dcplh in open-chan nel flow has a complication in that some types of channel cross sections do not always have a unique solution. This paper analyzes an alternative iterative pro cedure for quickly and accurately solving the implicit problem of determining the normal flow depth in complex channel sections. Conditions that guarantee a unique solution and guarantee that the iterative procedure will converge to the solution are developed. A computer program for quickly and accurately finding the unique solution, using the Chezy or Manning flow resistance equations, is available. Test runs for a rectangular, a triangular, a trapezoidal, and two complex channel cross sections are used to evaluate the effectiveness of the procedure. The test results show that the iterative procedure presented here meets the requirements of guar anteed convergence, computational efficiency (speed and accuracy), and the ability to handle both trapezoidal and complex channel cross sections.

A numerical procedure is developed for quickly and accurately finding the location and flow depth of a flow transition (critical control section) in open channels with discharge gradually varying along the channel length. General equations are presented for trapezoidal, triangular, and rectangular cross sections. A numerical analysis of the procedure shows that the geometry of the channel cross section is an important factor determining the existence of a unique solution to the flow-transition problem in open channel flow. A procedure is described to decide if solutions exist and, if they do, to compute them and select an acceptable solution as the starting point for water-surface profile calculations. Test runs on a computer are performed to verify the speed and accuracy of the proposed procedure. Examples involving the finding of location of flow transitions in open channels are used to illustrate the usefulness of the procedure. The findings from this study are beneficial to hydraulic engineers solving problems related to the design of lateral spillway channels and gutters for conveying stormwater runoff; to hydrologists using spatially varied flow equations to describe flow processes in natural channels; and to the enhancement of algorithms for spatially varied flow computations in hydrologic simulation models such as the CREAMS field-scale model and the WEPP watershed model.

A process based, distributed runoff erosion model (KINEROS2) was used to examine problems of parameter identification of sediment entrainment equations for small watersheds. Two multipliers were used to reflect the distributed nature of the sediment entrainment parameters: one multiplier for a raindrop induced entrainment parameter, and one multiplier for a flow induced entrainment parameter. The study was conducted in three parts. First, parameter identification was studied for simulated error free data sets where the parameter values were known. Second, the number of data points in the simulated sedigraphs was reduced to reflect the effect of temporal sampling frequency on parameter identification. Finally, event data from a small rangeland watershed were used to examine parameter identifiability when the parameter values are unknown. Results demonstrated that whereas unique multiplier values can be obtained for simulated error free data, unique parameter values could not be obtained for some event data. Unique multiplier values for raindrop induced entrainment and flow induced entrainment were found for events with greater than a two-year return period (~25 mm) that also had at least 10 mm of rain in ten minutes. It was also found that the three-minute sampling frequency used for the sediment sampler might be inadequate to identify parameters in some cases.

This paper evaluates the effects of watershed geometric representation (i.e., plane and channel representation) on runoff and sediment yield simulations in a semiarid rangeland watershed. A process based, spatially distributed runoff erosion model (KINEROS2) was used to explore four spatial representations of a 4.4 ha experimental watershed. The most complex representation included all 96 channel elements identifiable in the field. The least complex representation contained only five channel elements. It was concluded that oversimplified watershed representations greatly influence runoff and sediment yield simulations by inducing excessive infiltration on hillslopes and distorting runoff patterns and sediment fluxes. Runoff and sediment yield decrease systematically with decreasing complexity in watershed representation. However, less complex representations had less impact on runoff and sediment-yield simulations for small rainfall events. This study concludes that the selection of the appropriate level of watershed representation can have important theoretical and practical implications on runoff and sediment yield modeling in semiarid environments.