Effects of Vegetation Conversion upon Water Use by Riparian Plant Communities

摘要

Evapotranspiration was measured at two points simultaneously over a dense stand of saltcedar on the lower Colorado River with a computerized, portable Bowen ratio system. Twenty-one days of good ET data were collected during two successive years on eight different expeditions within the April-November growing season. In addition, potential ET (PET) estimates were developed for four models (radiation, radiation and temperature, net radiation and Penman combination) using climatological data from two weather stations at nearby Blythe, California. ET from the saltcedar ranged up above 12 mm per day in midsummer. The March 23-Nov. 11 growing season total was estimated from these measurements to be 1677 mm, including 40 mm of evaporated precipitation, and annual total ET was estimated to be 1727 mm. The climatological estimates of PET were converted to actual ET (AET) by a 4th-order calibrating function developed by regressing daily totals of measured Bowen ratio ET against estimated PET. Seasonal totals of AET were in excellent agreement and averaged 1756 mm. There was little difference between the two years. The two approaches suggest that 1750 and 1800 mm are reasonable figures for growing season ET and for annual ET from well-developed saltcedar on the lower Colorado. A simple radiation model appears suitable for estimating seasonal AET totals in this region if weather data are limited, and direct measurements of ET are lacking. The report describes the instrumentation, measurements and analysis techniques needed to develop measurements of AET from climatological estimates of PET.