Evapotranspiration and Greenup by Remote Sensing: An Analysis of the Colorado River Delta Following the Minute 319 Pulse Flow to Mexico

Abstract

In the spring of 2014, Mexico and the United States released 130 million cubic meters (mcm) of water to the lower Colorado River Delta (CRD) in Mexico as part of an agreement known as Minute 319. Once a perennial water flow, the CRD is now mostly dry due to upstream dams and diversions. The purpose of this environmental pulse flow was to examine the biologic and hydrologic response of this arid ecosystem. We used remote sensing techniques to assess vegetative health and condition in the time prior (2000-2013) and following (2014-2015) the pulse. Specifically, we used 250 m Moderate Resolution Spectroradiometer (MODIS) and 30 m Landsat 8 imagery to analyze two indicators of plant health and condition: greenup (based on the Normalized Difference Vegetation Index; NDVI) and evapotranspiration (ET). Using our ET estimates and salinity data collected from piezometers, we then developed a water balance model explaining the volume of shallow water entering this riparian system. We found that NDVI increased 17% in the year following the pulse and mean daily ET increased from 0.9 to 1.0 mm d-1. NDVI decreased in 2015, but was still significantly higher than pre-pulse (2013) levels. Based on our ET estimates and salinity data, we estimated ~ 103 mcm water enters the shallow riparian aquifer on an annual basis. Our results suggest that the deteriorated condition of vegetation within the CRD might not be reversed by a single pulse event and could instead require subsequent pulse flows as a long term strategy to restore vegetation in this riparian ecosystem.