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United States Department of Agriculture

Agricultural Research Service

Title: A Uniform Versus An Aggregated Water Balance of a Semi-Arid Watershed

Authors
item Flerchinger, Gerald
item Cooley, Keith
item Hanson, Clayton
item Seyfried, Mark
item Wight, J

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 10, 1997
Publication Date: N/A

Interpretive Summary: Hydrologists have long struggled with the problem of how to account for spatial variability in precipitation, vegetation and soils. Particular attention has recently been focused on methodologies to aggregate areas together to evaluate the areal response of hydrologic processes. In this study, two approaches were used to compute a water balance using two years of data collected at the Upper Sheep Creek Watershed, a 26-ha semi-arid mountainous sub-basin within the Reynolds Creek Experimental Watershed in southwestern Idaho. The approaches included: a uniform approach in which the entire watershed was assumed homogeneous; and an approach in which a water balance was computed for each landscape unit, then aggregated together to compute an overall water balance for the watershed. In the aggregated approach, the watershed was divided into three distinct areas according to dominant soils and vegetation. The water balance for the aggregated approach was within 3 and 51 mm, respectively for the two years while the uniform approach was within 22 and 88 mm, respectively. The larger errors in the uniform approach were attributed to inaccurate estimates of evapotranspiration as a result of not being able to associate available soil water with areas of the watershed with more vegetation, leaf area and potential to transpire the water. The error using the uniform approach amounts to approximately 5% of the total precipitation input for either year. For many applications, the increased effort required for the aggregated approach would not warrant the small increase in accuracy.

Technical Abstract: Hydrologists have long struggled with the problem of how to account for spatial variability in precipitation, vegetation and soils. Particular attention has recently been focused on methodologies to aggregate areas together to evaluate the areal response of hydrologic processes. In this study, two approaches were used to compute a water balance using two years of data collected at the Upper Sheep Creek Watershed, a 26-ha semi-arid mountainous sub-basin within the Reynolds Creek Experimental Watershed in southwestern Idaho. The approaches included: a uniform approach in which the entire watershed was assumed homogeneous; and an approach in which a water balance was computed for each landscape unit, then aggregated together to compute an overall water balance for the watershed. In the aggregated approach, the watershed was divided into three distinct areas according to dominant soils and vegetation. The water balance for the aggregated approach was within 3 and 51 mm, respectively for the two years while the uniform approach was within 22 and 88 mm, respectively. The larger errors in the uniform approach were attributed to inaccurate estimates of evapotranspiration as a result of not being able to associate available soil water with areas of the watershed with more vegetation, leaf area and potential to transpire the water. The error using the uniform approach amounts to approximately 5% of the total precipitation input for either year. For many applications, the increased effort required for the aggregated approach would not warrant the small increase in accuracy.

Last Modified: 9/21/2014
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