Effect of spatial and temporal variability of antecedent moisture content on model-generated runoff from an arid watershed
Creators: name: Meyer, William J.
Publisher: name: University of Nevada, Las Vegas
Year: 2010

description: Streams in the arid southwest are typically ephemeral, and stream gages are not commonly available. Consequently, runoff data from storm events is not available, and flood control facility design or other water resource related decisions are based on synthetic hydrographs. In the Mojave Desert region of Southern Nevada, the duration of storm used to develop these synthetic hydrographs is the 6 hour storm. The 6 hour storm is used to simulate high intensity summer storms. Additionally, soils information used in the calculations for these synthetic hydrographs is taken from maps that are generally developed for a broad range of issues and do not consider spatial or temporal variability in the hydraulic properties. Both the antecedent moisture content (AMC) as well as the hydraulic conductivity can vary due to a number of physical conditions that change within a specific soil type, and this variation can have a significant impact on the watershed runoff response. The objective of the research is to determine if calculated runoff volumes are due to the spatial variation of soil hydraulic properties, and which of these is more important. The thesis also addresses the importance of storm frequency as it relates to rainfall/runoff process and the spatial variation of soil hydraulic properties. A pedo-transfer function has been used for this study to develop the variation of the AMC parameter for the final hydraulic/hydrologic models. The effective hydraulic conductivity was evaluated at one half the calculated saturated hydraulic conductivity value. The AMC was varied for three different scenarios on 7 individual soil surfaces on a 228 km2 arid watershed in the Mojave Desert using a multi-model platform. The modeling software packages used for the thesis are Rosetta, HYDRUS-1D, ArcGIS, FLO-2D and HEC-1. All data were geospatially rectified on the watershed to account for the spatial variation of the soil parameters. Temporal variation of the water content on each soil surface was evaluated using atmospheric demand and the soil properties over a 30-day period. The results show that the spatial variability of hydraulic conductivity has a considerable effect on the calculated flowrates in a two-dimensional model (FLO-2D). Calculated flowrates for the upper watershed are shown to increase as storm frequency decreases. However, the 2 dimensional model shows that all rainfall and run-on from upstream infiltrates on the lower portion of the watershed. The results also show that the AMC within a watershed has a considerable effect on calculated flowrates for both a two-dimensional and a one-dimensional model. As the time period of the drying time increases from 1-day to 5-days, the calculated runoff reduces by 42 percent to 250 percent in FLO-2D (at the upper watershed) and by 3600 percent and 4200 percent in HEC-1 for the 100-year models. There was a large difference in calculated peak flowrates between the one-dimensional and 2-dimensional models indicating a considerable difference between lumped parameter and discretized modeling methods.