Abstract
The accurate estimation of water use by groundwater-dependent riparian vegetation is of great importance to sustainable water resource management in arid/semi-arid regions. Remote sensing methods can be effective in this regard, as they capture the inherent spatial variability in riparian ecosystems. The single-satellite-scene (SSS) method uses a derivation of the Normalized Difference Vegetation Index (NDVI) from a single space-borne image during the peak growing season and minimal ground-based meteorological data to estimate the annual riparian water use on a distributed basis. This method was applied to a riparian ecosystem dominated by tamarisk along a section of the lower Colorado River in southern California. The results were compared against the estimates of a previously validated remotely sensed energy balance model for the year 2008 at two different spatial scales. A pixel-wide comparison showed good correlation (R2 = 0.86), with a mean residual error of less than 104 mm∙year−1 (18%). This error reduced to less than 95 mm∙year−1 (15%) when larger areas were used in comparisons. In addition, the accuracy improved significantly when areas with no and low vegetation cover were excluded from the analysis. The SSS method was then applied to estimate the riparian water use for a 23-year period (1988–2010). The average annual water use over this period was 748 mm∙year−1 for the entire study area, with large spatial variability depending on vegetation density. Comparisons with two independent water use estimates showed significant differences. The MODIS evapotranspiration product (MOD16) was 82% smaller, and the crop-coefficient approach employed by the US Bureau of Reclamation was 96% larger, than that from the SSS method on average.
Highlights
Large extents of the Colorado River floodplain are currently occupied by invasive species, such as tamarisk or salt cedar (Tamarix spp.) and Russian olive (Eleagnus angustifolia), that have replaced the native species, such as cottonwood (Populus spp.) and willow (Salix spp.)
The majority of points in the lower-left cluster in Figure 4 were from subareas 1, 2, and 3, where vegetation was sparse, with average Normalized Difference Vegetation Index (NDVI) less than 0.31 for all three day of year yearduring (DOY)
The evaluation of SSS performance was conducted at the area-wide scale, where a comparison of the single-satellite-scene evapotranspiration (SSS-ET) and remotely sensed energy balance (RSEB)-ET was made on all DOYs (195, 227, 259) over the six subareas within the Cibola National Wildlife Refuge (CNWR)
Summary
Large extents of the Colorado River floodplain are currently occupied by invasive species, such as tamarisk or salt cedar (Tamarix spp.) and Russian olive (Eleagnus angustifolia), that have replaced the native species, such as cottonwood (Populus spp.) and willow (Salix spp.). MODIS and the Blaney–Criddle reference ET (ETo-BC : [33]) This new model had reduced error (within 20%) when applied to riparian and agricultural areas along the Lower Colorado River in the southwestern U.S In a more recent study, [29] replaced ETo-BC with the Penman–Monteith ETo [34], and developed an exponential relation. This newer method had a better performance in predicting ET, with an error of 10% when compared with flux tower and water balance data from riparian zones and irrigation districts at multiple locations from western U.S, Spain, and Australia [29]. ET variations across parts of the Cibola National Wildlife Refuge in the Lower Colorado River Basin
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