1. Water Balance Model Ted Sammis

2. Water Balance I+R-D-Et+- del SM=0 I+R-D-Et+- del SM=0 I= irrigation inches 1-4 inches I= irrigation inches 1-4 inches R= rainfall inches = 0-1 inches R= rainfall inches = 0-1 inches Et= evapotranspiration 0.4 inches/day Et= evapotranspiration 0.4 inches/day Del SM= change in soil moisture inches Del SM= change in soil moisture inches Time step 1 day Time step 1 day

3. I+R-D-Et+- del SM=0 I+R-D-Et+- del SM=0 Del SM= SM i - SM i-1 Del SM= SM i - SM i-1 Substitute into equation Substitute into equation SM i = I+R-D-Et+SM i-1 SM i = I+R-D-Et+SM i-1 Et= kc Et o Et= kc Et o SM i = I+R-D- kc Et o +SM i-1 SM i = I+R-D- kc Et o +SM i-1

4. Example I=0 R=0 D=0 kc=1 Et o = 0.4 inches FC= 0.3 inches/inch of soil Rd= 36 inches I=0 R=0 D=0 kc=1 Et o = 0.4 inches FC= 0.3 inches/inch of soil Rd= 36 inches Start at field capacity Start at field capacity SM i-1 = Pv Rd= 0.3*36=10.8 inches SM i-1 = Pv Rd= 0.3*36=10.8 inches SM i = I+R-D- kc Et o +SM i-1 SM i = I+R-D- kc Et o +SM i-1 SM i = 0+0-0-1*0.4+10.8= 10.4 inches SM i = 0+0-0-1*0.4+10.8= 10.4 inches

5. Example day two I=0.0 R=0.1 D=0 kc=1 Et o = 0.4 inches SM i-1 =10.4 inches I=0.0 R=0.1 D=0 kc=1 Et o = 0.4 inches SM i-1 =10.4 inches SM i = I+R-D- kc Et o +SM i-1 SM i = I+R-D- kc Et o +SM i-1 SM i = 0.0+0.1-0-1*0.4+10.4= 10.1 inches SM i = 0.0+0.1-0-1*0.4+10.4= 10.1 inches

6. Example day three I=1.5 R=0.0 D=0 kc=1 Et o = 0.4 inches I=1.5 R=0.0 D=0 kc=1 Et o = 0.4 inches SM i-1 =10.1 inches SM i-1 =10.1 inches SM i = I+R-D- kc Et o +SM i-1 SM i = I+R-D- kc Et o +SM i-1 SM i = 1.5+0-0-1*0.4+10.1= 11.2 inches SM i = 1.5+0-0-1*0.4+10.1= 11.2 inches SM i is greater than Field Capacity=10.8 SM i is greater than Field Capacity=10.8 Reset SM i to 10.8 and D= 11.2-10.8=0.4 Reset SM i to 10.8 and D= 11.2-10.8=0.4 Check calculation Check calculation SM i = 1.5+0-0.4-1*0.4+10.1= 10.8 inches SM i = 1.5+0-0.4-1*0.4+10.1= 10.8 inches

7. Water Balance Water balance gets reset every time irrigation is enough to fill the root zone to field capacity. Water balance gets reset every time irrigation is enough to fill the root zone to field capacity. Error in Et accumulates only between irrigations then resets. Error in Et accumulates only between irrigations then resets.