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