MODIS Vegetation Metrics as Indicators of Hydrological Response in Watersheds of California Mediterranean-type Climate Zones

Vegetation characteristics of a watershed can be important in determining hydrological response variables (HRVs) such as streamflow (Q), evapotranspiration (ET), and river yield (Q/P). Quantifying the relationship between satellite-derived vegetation metrics and hydrological response to precipitation (P) has the potential to aid in the prediction of streamflow and evapotranspiration for ungauged watersheds. The utility of Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data to estimate HRVs of watersheds at the regional scale (southern and central California) is tested in this study. An exhaustive statistical regression analysis was conducted, to quantify the relationship between MODIS vegetation metrics and HRVs. Both ordinary least squares and spatially varying parameter models were tested.

Moderate linear relationships were found between several MODIS-derived vegetation metrics and both ET and Q. MODIS Leaf Area Index (LAI) regressed on ET exhibited the strongest relationship regression coefficients of 0.50 and 0.95 for the study period mean and strongest annual mean, respectively. Results indicate that the inclusion of spatial-varying parameters can improve the fit in the relationship between MODIS vegetation metrics and HRVs relative to that of the traditional fixed parameter model.

The use of a relatively novel exhaustive statistical approach to testing multiple watersheds with varying land cover and climatic characteristics shows that drought conditions in the southern watersheds (2002 and 2004) yielded the strongest fits between MODIS vegetation metrics and HRVs. Alternatively, the relationship between MODIS vegetation metrics and HRVs was weaker when sampling watersheds from similar climatic zones (i.e. humid or semi-arid). Results suggest that direct region-wide estimates of annual streamflow and evapotranspiration for ungauged watersheds can be made reliably for drier years, and that MODIS-derived vegetation metrics may be useful for aiding identification of hydrologically similar watersheds for model-based regionalization purposes.