Prediction of Biome-Specific Potential Evapotranspiration in Mongolia under a Scarcity of Weather Data

We propose practical guidelines to predict biome-specific potential evapotranspiration (<i>ET_p</i>) from the knowledge of grass-reference evapotranspiration (<i>ET</i>₀) and a crop coefficient (<i>K_c</i>) in Mongolia. A paucity of land-based weather data hampers use of the Penman–Monteith equation (FAO-56 PM) based on the Food and Agriculture Organization (FAO) guidelines to predict daily <i>ET</i>₀. We found that the application of the Hargreaves equation provides <i>ET</i>₀ estimates very similar to those from the FAO-56 PM approach. The <i>K_c</i> value is tabulated only for crops in the FAO-56 guidelines but is unavailable for steppe grasslands. Therefore, we proposed a new crop coefficient, <i>K_{c adj}</i> defined by (a) net solar radiation in the Gobi Desert (<i>K_{c adjD}</i>) or (b) leaf area index in the steppe region (<i>K_{c adjS}</i>) in Mongolia. The mean annual <i>ET_p</i> obtained using our approach was compared to that obtained by FAO-56 guidelines for forages (not steppe) based on tabulated <i>K_c</i> values in 41 locations in Mongolia. We found the differences are acceptable (<i>RMSE</i> of 0.40 mm d⁻¹) in northern Mongolia under high vegetation cover but rather high (<i>RMSE</i> of 1.69 and 2.65 mm d⁻¹) in central and southern Mongolia. The FAO aridity index (<i>AI</i>) is empirically related to the <i>ET_p</i>/<i>ET</i>₀ ratio. Approximately 80% and 54% reduction of <i>ET</i>₀ was reported in the Gobi Desert and in the steppe locations, respectively. Our proposed <i>K_{c adj}</i> can be further improved by considering local weather data and plant phenological characteristics.