Month J F M A M J A S 0 N D 145 107 95 64 43 37 20 28 45 88 152 149 13 18 31 48 77 99 114 105 74 47 39 16 PRECIP (mm) POTET (mm) PRECIP POTET ASTRGE 1132896416 164 16 -34 -34-62-94 -77 29 41 1/3 133 SM ACTET DEFIC SURPL Soil Moisture Index Step 1 Calculate the difference between PRECIP and POTET and record this value in row 3 of the water budget table. Step 2 The change in soil moisture and the total soil moisture storage (rows 4 & 5) should be calculated at the same time. When the soil is fully saturated in this case soil capacity is arbitrarily set at 80 mm) and PRECIP exceeds POTET then the change in soil moisture (ASTRGE) will be zero (the soil is fully saturated and all additional precipitation becomes runoff). If the soil moisture is not at capacity (when soil moisture is below 80 mm) then precipitation (PRECIP) is allocated to soil moisture until either all that month's precipitation is accounted for or the soil reaches its capacity for storage (moisture beyond the soils storage capacity is recorded as surplus for the month). If PRECIP is not sufficient to meet the POTET demand then moisture will be removed from the soil until POTET is met or the soil moisture is zero. Note that soil moisture can not be negative and that the previous month's soil moisture is used to calculate the change in storage (ASTRGE) when performing the calculations. Step 3 Calculate the ACTET (Row 6) using the data in the rows above it. ACTET will equal POTET as long as the total available moisture (PRECIP+SM) is equal to or greater than the potential for evapotranspiration (POTET). Calculate ACTET and enter it in row 6 of your water budget table. Step 4 Calculate the deficit, if one exists, by subtracting ACTET from POTET. Water budget deficits exist where insufficient water is available, either in the form of precipitation or water stored in the soil, for meeting the system's full potential for evapotranspiration. Therefore, if ACTET is less than POTET then a deficit (DEFIC) exists and should be calculated for entry in row 7 of your water budget table. Step 5 Calculate the surplus (SURPL) if one exists. A surplus is a condition where the full POTET is met and the soil has been fully saturated. This represents water which becomes runoff. All moisture in a given month should first be assigned to first meeting ACTET and to recharging the soil, any moisture which remains in a given month once these two demands are fully met is surplus and should be entered in row 8. Step 6 Calculate the Soil Moisture Index and enter it into row 9 of your water budget table. To calculate the Index number, divide the PRECIP-POTET value by the POTET value and enter this result, along with the sign indicating its positive or negative value in row 9