Reference ET: The global 1-degree reference ET (ETo), based on the 6-hourly Global Data Assimilation Systems (GDAS) model output, is calculated daily at EROS (Senay and Verdin [11]). The GDAS ETo (Figure 4) uses the standard Penman-Monteith equation as outlined in the FAO publication for short-grass ETo by Allen et al. [12]. The feasibility of using the GDAS ETo for such applications was recommended by Senay and Verdin [11] after a comparison with station-based daily ETo showed encouraging results with r2 values exceeding 0.90. Daily global ETo values were available for all days between 2001 and 2005. For 2000, the daily ETo values were not complete. For the missing time periods, the average daily ETo from 2001 to 2005 was used.