Abstract
We measured the evaporative fraction (EF) across a semiarid elevation gradient in the San Jacinto and Santa Rosa Mountains of southern California using the Regional Evaporative Fraction Energy Balance platform and four eddy covariance towers. We compared our measurements to precipitation estimates and satellite observations of vegetation indices to assess the seasonal and interannual controls of precipitation and vegetation on surface energy exchanges. Precipitation amount and timing had the largest effect on evaporative fraction, with vegetation having a relatively greater importance at higher elevations than lower elevations. Vegetation cover was linearly related to mean annual EF, but did not predict seasonal variation in EF in most of the study area's ecosystems. Multiyear vegetation observations show that vegetation density increases in a stepwise pattern with precipitation, probably due to shifts in dominant plant communities. Precipitation is a more important factor in controlling EF than temperature. Possible future climate change, including decreases in precipitation amount and increases in variability, could decrease vegetation cover, thus reducing EF and increasing albedo.
Highlights
Ecosystem shifts in southern California’s mountains are likely to be of economic and societal importance. These mountains lie at “urban‐wildland fire interfaces,” where human activities have increased the frequency of wildfire [Syphard et al, 2007], which can act as agent of ecosystem change [Scheffer et al, 2001]
We evaluated vegetation indices’ capability to predict ecosystem evaporative fraction (EF) at various timescales
Satellite approaches to estimate regional evapotranspiration do not work well because they rely on gradients of radiometric temperatures and/or vegetation indices to estimate evaporative fraction (EF) and Evapotranspiration (ET) based on empirical relationships; these relationships are confounded by elevation effects [Li et al, 2009]
Summary
[2] Patterns of vegetation distribution and energy exchange are crucial because they control regional climate [Beringer et al, 2005; Hogg et al, 2000; Wendt et al, 2007], ecosystem water yield [Huxman et al, 2005; Molotch et al, 2009], and are closely linked to ecosystem productivity [Law et al, 2002; Veenendaal et al, 2004; Scott et al, 2009]. These mountains lie at “urban‐wildland fire interfaces,” where human activities have increased the frequency of wildfire [Syphard et al, 2007], which can act as agent of ecosystem change [Scheffer et al, 2001] Urbanization near these mountains may have already lead to regional temperature increases [LaDochy et al, 2007] that could affect mountain evaporation and/or energy exchange by increasing potential evapotranspiration. Satellite approaches to estimate regional evapotranspiration do not work well because they rely on gradients of radiometric temperatures and/or vegetation indices to estimate evaporative fraction (EF) and Evapotranspiration (ET) based on empirical relationships; these relationships are confounded by elevation effects [Li et al, 2009] Used approaches, such as the Temperature‐Vegetation Triangle Index or surface energy balance algorithms, require flat surfaces [e.g., Bastiaanssen et al, 1998; Carlson, 2007]. Larger‐scale sampling approaches such as scintillometry [Ezzahar et al, 2007; Kleissl et al, 2009] can assess wider spatial averages of latent heat fluxes from residuals of net radiation, ground heat flux, and sensible heat flux, but these methods obscure spatial patterns
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