EVAPOTRANSPIRATION: THE MISSING LINK IN HYDROLOGIC CYCLE ANALYSIS OF THE REPUBLIC OF THE MARSHALL ISLANDS (RMI)

Freshwater resource scarcity is one of the most pressing consequences associated with global climate change. In the Republic of the Marshall Islands (RMI) future freshwater security is threatened by changing precipitation patterns and sea level rise. The RMI comprises numerous low lying atolls in two parallel island chains that spread across approximately 70 square miles and experience diverse climates ranging from dry northern atolls to wetter southern atolls. The coral-derived soils on these atolls allow salt-water intrusion into fresh groundwater lenses, thus degrading one potential water source. To better understand the freshwater budget of these atolls, we sought to develop a simple method for quantifying the potential evapotranspiration (PET) on the atolls based on readily available climate data and satellite imagery. We tested our methods using two atolls with contrasting amounts of rainfall, Likiep (North, dry) and Ebon (South, wet).

We established a three-step process for calculating atoll-wide PET rates. First we used a combination of high resolution satellite imagery and literature review to determine the major plant types on each of the atolls. We then used the Maximum Likelihood Classification (MLC) spatial analyst tool in ArcGIS to quantify the spatial distribution of these plants on each atoll. We used pixel counts to determine the fraction of the atoll covered by each of the main plant types. Likiep comprises approximately 42% coconut trees and 58% grass cover, whereas Ebon is primarily covered by coconut (approximately 98%) with only 2% grass. Second, we calculated crop evapotranspiration (ET_c) based on available regional climate data using the CropWAT tool and adjusted by a crop coefficient (K_c). Lastly, the major plant type distributions were combined with the calculated ET_c for each plant type to determine atoll-wide PET. Results indicate that PET for Likiep is 3.64 + 0.27 mm/day and for Ebon is 2.86 + 0.21 mm/day.