|Woolhiser, D. - ARS RETIRED|
|Shillito, R. - UNIV. OF ARIZ.|
|Miller, S. - UNIV. OF ARIZ.|
|Syed, K. - UNIV. OF ARIZ.|
Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 3, 1997
Publication Date: N/A
Interpretive Summary: Runoff waters are very important in semiarid regions for several reasons. First, they provide a vital resource to sustain human habitation and agricultural production. However, runoff in semiarid regions is often very rapid and can result in considerable flood damage. A study was carried out in southeastern Arizona using information from 29 watersheds to investigate how runoff response changes from small to large watersheds ranging in size from 0.18 to 14800 hectares (0.44 to 36,600 acres). It was found that there was a distinct change in the nature runoff response for watersheds smaller than about 60 hectares (~150 ac) to those larger than this drainage area. For the larger watersheds, the annual runoff water yield and peak floods increase at a much smaller rate as the drainage area increases than for the smaller watersheds. This was explained by increasing runoff losses in the typically dry semiarid river beds and the rainfall storms covering a smaller portion of the larger watersheds. This finding has important implications for sizing structures to capture and control runoff waters for subsequent use and for using computer models to predict runoff in arid and semiarid regions.
Technical Abstract: The non-linear nature of watershed runoff response for a given watershed area has been recognized for some time. However, the literature suggests that watershed response becomes more linear with increasing scale due to increased averaging at larger drainage areas and dominance of response by channel flow. This hypothesis was tested using observed data from a series sof 29 nested watersheds and modeling studies with a subset of these watersheds within the USDA-ARS Walnut Gulch Watershed in southeastern Arizona. In addition to analyses of observed rainfall, runoff, and physiographic data two types of distributed rainfall-runoff models were applied to a subset of the watersheds to further investigate the scale dependent nature of the collected data and to assess its impacts on flood-f response in this ephemeral watershed becomes more non-linear with both increasing watershed scale and decreasing storm size. This occurs primarily due to the increasing effect of channel losses and associated infiltration as well as a rapid decrease in the fraction of watershed area covered by storms as drainage area increases.