Grid-scale evaluation of five reference evapotranspiration methods based
on the climate forecast system reanalysis data
Abstract
Climate Forecast System Reanalysis data offer a promising database for
overcoming the limitations in availability and reliability of
climatological data and, hence, for understanding the hydrological
processes. Using these data on grid-by-grid, seasonal and yearly scales,
the present study attempts to advance the spatiotemporal evaluation of
two radiation-based (Priestley–Taylor and Makkink) and three
temperature-based (Hargreaves–Samani, Thornthwaite and
Blaney–Criddle), against estimates of grass reference
evapotranspiration (ETo) by FAO Penman-Monteith method (FAO-PM). The
analysis was performed for the period 1979–2013, considering the second
largest (79,000 km2) river system in Ethiopia, i.e. Omo-Gibe basin,
which accommodates national parks and vast hydropower, cultivation and
afforestation developments and discharges its flow to Lake Turkan in
Kenya. To comprehensively explain the pattern of PET, the influences of
temperatures, rainfall, wind speed, radiation, relative humidity and
elevation on PET were also examined. The results emphasize the
outperformance of Hargreaves-Samani method. In overall, both the annual
and seasonal FAO-PM estimates are captured by this method for most of
the grid locations. Annual trends in ETo in the upper region increased
but rainfall trends decreased. These trends might negatively impact the
rain-fed food production by reducing soil moisture availability in the
river basin. Comparatively, trends in rainfall in the middle and lower
regions increased with a higher magnitude while ETo increased with a
smaller magnitude compared. The above-mentioned trends in ETo are
attributable to rising temperature and decreasing relative humidity,
wind speed, and solar radiation, respectively. If these trends would
continue, we would expect increase in soil moisture for sugarcane
plantation in the middle and lower region and attenuation of water loss
from reservoirs in the river basin. This study improves the
understanding of the best potential evapotranspiration methods in
similar data-scarce river basins in Ethiopia or other transboundary
rivers in the region or worldwide.