Historical changes in the area of riparian woodland and maximum ground-water evapotranspiration (GWET) rates were estimated and used in a regional ground-water-flow model of a 78,000 square-kilometer area encompassing the upper Republican River Basin. Documentation of the historical change in area of riparian woodland and transpiration rates for the cottonwood-dominated woodland was not available for the time period and location examined. The purpose of this riparian study was to improve estimates of actual GWET on a regional scale.

Aerial photographs and digital orthophoto quadrangles were analyzed to estimate historical expansion of riparian woodland in the upper Republican River Basin. The photographs were georeferenced to the digital orthophoto quadrangles using a minimum of 16 ground-control points with the root-mean-square error less than 1 meter. Canopy area was delineated from the imagery at 16 sites in the study area, located above and below the seven major reservoirs and on two streams without reservoirs. Three time periods between 1937 and 1998 were examined, representing conditions before, less than 1 decade after, and 3 to 4 decades after reservoir construction. The area-weighted average change in woodland area estimated for the 16 reaches indicated that 1937-38 woodland area increased about 380 percent by the 1990s.

Maximum GWET rates (per area) by the riparian woodlands were estimated as the difference between potential transpiration and actual transpiration from the soil zone, which was simulated using a soil-water balance model with 116 unique combinations of soil type and climatic stations. For these calculations it was assumed that phreatophytic trees will use all the soil water that can be feasibly extracted, and in soil-water limited periods will have unlimited access to shallow ground water and thus will transpire at the potential rate. The maximum rate would only occur where the depth to water was small. The maximum annual GWET volumetric flux for the 16 sites was calculated as the product of the area of woody riparian vegetation and the maximum rate per area. Because GWET rates changed little through time, nearly four-fold increases in woodland area resulted in proportional increases in maximum GWET fluxes.