Document Type

Report

Publisher

Utah. Governor's Public Lands Policy Coordinating Office

Publication Date

9-4-2008

First Page

1

Last Page

163

Abstract

The amount of water produced from a watershed depends on the climate, soils, geology, land cover and land use. Precipitation water inputs in the form of rain or snow are partitioned by the watershed into evapotranspiration, runoff and groundwater recharge. This study has examined factors that may impact the production of runoff from Utah watersheds, focusing on factors related to land and watershed management. Specifically we are interested in how land use changes, such as afforestation, deforestation, agricultural, urban, industrial and mining development, impact runoff. The scale of interest is regional subbasins at the USGS cataloging unit 8 digit Hydrologic Unit Code (HUC) scale (http://water.usgs.gov/GIS/huc.html). Twelve 8 digit HUCs in Utah, with an average area of 4500 km2 were selected for this study. Within these subbasins we identified a total of 39 watersheds draining to USGS streamflow gages, chosen either from the USGS Hydroclimatic Climatic Data Network of gages that are minimally impacted by anthropogenic alterations, or to be representative of large areas within the chosen HUCs with long relatively continuous streamflow records. In each of these watersheds we examined trends in precipitation, temperature, snow, streamflow and runoff ratio. Runoff ratio is the fraction of precipitation that becomes streamflow. We also examined land use and land cover information for these watersheds from the national land cover dataset, southwest regional GAP analyses and the Utah division of water resources water related land use inventory. The most consistent trend noted was in temperature which is increasing. We did not note any significant trends in precipitation. Fourteen of the 39 watersheds examined had significant decreasing trends in streamflow and runoff ratio. We were unable to find definitive causes for these streamflow and runoff ratio trends, though we do have indications that some of them are associated with human development, storage in reservoirs and land cover and land use changes. In analysis of the land cover data we found that unequivocal interpretation of land cover changes was confounded by differences in methodology and technology used to determine land cover over time. We were consequently unable to derive relationships from the data as to how land cover and land use affect water production. So as to provide some information helpful for land management policy making and economic analyses we developed a water balance approach that quantifies sensitivity of runoff production to changes in land cover based on differences in evapotranspiration from different land cover types. The coefficients that quantify the potential evapotranspiration from each land cover type in this analysis are based on our judgment and information from the literature. In coming up with these coefficients we also endeavored to reconcile them with precipitation, streamflow and runoff ratio data for the Utah study watersheds. This water balance approach provides predictions of how water production from these Utah watersheds may change with land cover changes. By considering a range of water balance model parameters we provide water balance derived bounds on how streamflow could change given land cover changes. However, we caution that in the use of these results, the sensitivities depend directly upon the coefficients that quantify the potential evapotranspiration from each land cover type. This represents a fairly gross simplification. In semi-arid settings the vegetation water use is often limited by water availability, rather than potential evapotranspiration, and differences in water yield may relate more to factors such as the timing and rate of water inputs (precipitation intensity and snowmelt). Vegetation also depends strongly on topographic setting, due to factors such as elevation, aspect, and solar radiation exposure. Changes in the proportioning of land cover in a watershed therefore should consider the control that topographic setting has on land cover. Economic considerations associated with changes in water production were also examined. Value was estimated using two approaches: (1) the price for leases and sales of water rights, and (2) using "shadow values" derived from economic models based on the increasing profitability of water users as water availability increases (or decreases). In general, we noted that irrigated agriculture was responsible for around 80% of water diversions, but that municipal and industrial (M&I) water had higher prices than water for irrigated agriculture. Purchase price for irrigated agriculture in this region has ranged from $25 to $400 per acre foot. Purchase price for M&I water has ranged from $300 to $25,000 per acre foot. Shadow value estimates of the value of water to irrigated agriculture ranged from $300 to $1,500 per acre foot. These figures indicate that, while the value of water is time and place dependent, and it is difficult to generalize about the value of the changes in water production, that in general the economic value of additional water in irrigation is relatively low, but that water for M&I is generally of higher economic value, which suggests that increases in water production from watersheds serving urban areas are likely to have relatively high returns, while water increases used for irrigation use will have relatively low returns.

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