Abstract
Large quantities of water are consumed by irrigated crops and riparian vegetation in western U.S. irrigation districts. Remote sensing methods for estimating evaporative water losses by soil and vegetation (evapotranspiration, ET) over wide river stretches are needed to allocate water for agricultural and environmental needs. We used the Enhanced Vegetation Index (EVI) from MODIS sensors on the Terra satellite to scale ET over agricultural and riparian areas along the Lower Colorado River in the southwestern U.S., using a linear regression equation between ET of riparian plants and alfalfa measured on the ground, and meteorological and remote sensing data, with an error or uncertainty of about 20%. The algorithm was applied to irrigation districts and riparian areas from Lake Mead to the U.S./Mexico border. The results for agricultural crops were similar to results produced by crop coefficients developed for the irrigation districts along the river. However, riparian ET was only half as great as crop coefficient estimates set by expert opinion, equal to about 40% of reference crop evapotranspiration. Based on reported acreages in 2007, agricultural crops (146,473 ha) consumed 2.2 × 109 m3 yr−1 of water. All riparian shrubs and trees (47,014 ha) consumed 3.8 × 108 m3 yr−1, of which saltcedar, the dominant riparian shrub (25,044 ha), consumed 1.8 × 108 m3 yr−1, about 1% of the annual flow of the river. This method could supplement existing protocols for estimating ET by providing an estimate based on the actual state of the canopy as determined by frequent-return satellite data.
Highlights
Need for Wide Area Estimates of Riparian and Agricultural Water Use on Arid Zone RiversThroughout the arid zones, there is conflict between human and environmental water needs [1,2].The major rivers are often fully utilized for human uses, and in-stream flows have been diminished
There is concern that it uses large amounts of water compared to native plants [8,9], and the U.S Congress passed a bill to conduct saltcedar and Russian olive (Elaeagnus angustifolia) control programs on western rivers for the purpose of water salvage [10]
Plots of MODIS ET calculated by Equation (4) and ground ET estimates are in Figure 2 and Figure 3
Summary
Need for Wide Area Estimates of Riparian and Agricultural Water Use on Arid Zone RiversThroughout the arid zones, there is conflict between human and environmental water needs [1,2].The major rivers are often fully utilized for human uses, and in-stream flows have been diminished. Need for Wide Area Estimates of Riparian and Agricultural Water Use on Arid Zone Rivers. U.S river managers are challenged to manage available water supplies for both human use, primarily agriculture, and environmental restoration [3,4], and for this they need accurate estimates of consumptive water use (evapotranspiration, ET) by crops and riparian species. There is concern that it uses large amounts of water compared to native plants (early ET estimates by indirect methods were as high as 3–4 m yr−1) [8,9], and the U.S Congress passed a bill to conduct saltcedar and Russian olive (Elaeagnus angustifolia) control programs on western rivers for the purpose of water salvage [10]. Resolving differences in estimates of water consumption by saltcedar and other riparian species over long river reaches is important in constructing water budgets and developing riparian management plans for western river irrigation districts
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