Monitoring the Lower Colorado River's Arid Delta in Mexico by Measuring the Response in Vegetation and Evapotranspiration Resulting from the 2014 Spring Pulse Flood

The Lower Colorado River Spring 2014 pulse flood release of water to the delta in Mexico is a collaborative monitoring project funded by the Department of Interior, in part, with teams of scientists from governments, universities and non-profits on both sides of the border. Our goal was to provide measures of the vegetation response to this Minute 319 pulse flood and to document post-flooding changes in the vegetation along the lower Colorado River reaches 1-7, which include ca. 150 narrow miles of riparian habitat until it opens to the Sea of Cortez.

We used Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI) from the Moderate Resolution Imaging Spectrometer (MODIS) on the Terra satellite, which provides near-daily coverage at 250 m resolution, while the Landsat 8 satellite provides this data at 16 day intervals at 30 m resolution. We are combining the two sources of satellite data to obtain high spatial and temporal resolution. NDVI and EVI data for each river reach from 2000 to the present were collected, as well as VI for specific target areas. These include restoration sites, vegetation transect sites, and bird observation sites.

Green vegetation has decreased steadily in all river reaches since the flood years of 1997-2000. This loss of vegetation vigor has been accompanied by a loss of habitat for riparian dependent birds from 2002 to the present. The loss of vegetation vigor resulted in a lowering of evapotranspiration (ET) in each river reach. ET has decreased approximately from 155 mcm/year in 2000 to 100 mcm/year in 2013. The pulse flood, at 130 mcm, is designed to restore some of the vegetation vigor and to germinate new cohorts of native trees throughout the river reaches. Early positive results are apparent in the zones of inundation. For example, an area of about 600 hectares has shown rapid green-up at the end of the pilot channel in Reach 5 and extending into Reach 7. This is a mixed vegetation zone containing native trees, saltcedar (Tamarix sp.)  and other riparian species. In the longer run, groundwater recharged upstream is expected to restore further productivity in downstream river reaches. We document here that results of a pulse flood plays out over a period of years in terms of enhanced productivity of the groundwater-dependent riparian vegetation.