ARS | Publication request: Long-term water balance and conceptual model of a semi-arid mountainous catchment

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 22, 2011
Publication Date: April 1, 2011
Citation: Chauvin, G., Flerchinger, G.N., Link, T.E., Marks, D.G., Winstral, A.H., Seyfried, M.S. 2011. Long-term water balance and conceptual model of a semi-arid mountainous catchment. Journal of Hydrology. 400:133-143.

Interpretive Summary: Reintroducing fire as part of the natural cycle to control invasive weeds and improve habitat on rangeland is becoming an accepted practice, however post-fire vegetation, water balance and streamflow responses are not well understood. This study characterizes the pre-fire annual water balance over 23 years of observations within the Upper Sheep Creek Watershed, a small mountainous rangeland watershed within the Reynolds Creek Experimental Watershed. A prescribed fire was conducted in the Upper Sheep Creek Watershed in the fall of 2007. This pre-fire analysis will serve as a baseline for comparison of the post-fire hydrology, water balance and streamflow. Comparison of pre- and post-fire hydrologic process will lead to a better understanding of the immediate and long-term effects of prescribed fire on the hydrology, water balance and streamflow of mountainous rangeland watersheds.

Technical Abstract: Long-term water balance investigations are needed to better understand hydrologic systems, especially semi-arid mountainous catchments. These systems exhibit considerable interannual variability in precipitation as well as spatial variation in snow accumulation, soils, and vegetation. This study extended a previous 10-year water balance (water years 1985-1994) to 23 years (water years 1984-2006) for the Upper Sheep Creek (USC) catchment, a 26 ha, snow-fed, semi-arid rangeland headwater drainage within the Reynolds Creek Experimental Watershed in southwestern Idaho, USA. The current study captured 11 additional years with runoff, resulting in 70% of the years in the study having runoff as compared to only 50% for the ten-year study. The average annual effective precipitation for the USC catchment over the 23-year study period was 547 mm, which is 16% higher than the average annual effective precipitation of the ten-year study (471 mm). Evapotranspiration (ET) averages increased only slightly from the ten-year study to 23-year study, suggesting that a fixed amount of moisture is made available to vegetation at the beginning of the growing season when soil moisture approaches field capacity. ET amounts from the sagebrush areas differed dramatically from year to year, while those for the aspen area were more consistent annually. The ten-year water balance study at USC identified that runoff from the catchment was correlated (r2 = 0.52) to precipitation above a critical threshold of approximately 450 mm of precipitation necessary to generate runoff and that a strong relationship existed between simulated percolation beyond the root zone and measured runoff. The results from the current long-term study, however, indicate that associated thresholds and trends between these hydrologic quantities are strongly dependent on both antecedent and current hydroclimatic conditions. The influence of the system variability on runoff was captured reasonably well by simulations performed by the SHAW model, with r2 between runoff and simulated percolation beyond the root-zone as high as 0.96. This study will serve as a basis for post-treatment comparison for USC where a prescribed fire was conducted in the fall of 2007.