1. 1 Delta Island Agriculture Technical Workshop on Modeling Issues of the Delta February 6, 2007 Jim Wilde, P.E. Delta Modeling Section California Department of Water Resources wildej@water.ca.gov

2. 2 Discussion Topics Delta Island Consumptive Use (DICU) –Model description DICU model results –How does the model do? DICU uncertainties –What are the inherent model uncertainties

3. 3 DICU Model Inputs 142 subareas Some islands contain > 1 subarea 20 land use types Each type is aggregated for each subarea 7 precipitation stations Distributed by Thiessen Polygon method Constant seepage into low lands 0.3 inches per foot of crop root depth per month Long term average crop consumptive use CU is then adjusted by pan evaporation

4. 4 DICU Model Outputs Diversions –Water taken out of channels Seepage –Channel loss to the islands Drains –Return water pumped back to channels Monthly Average:

5. 5 Ag Diversions and Drains

6. 6 Impact of Ag Use in Dry Periods

7. 7 DICU Model Uncertainty Irrigation –Fixed seasons for each crop –Schedule differs for critical and non-critical water year types Monthly time step reduces variability –Stormwater runoff cannot be estimated Long term historic land use patterns not used –The land use is assumed constant –Double cropping not considered

8. 8 DICU Model Uncertainty Leaching patterns are assumed the same every year –Applied as a post-processing step CU adjustment from one pan evaporation location –Uncertainty in spatial variability What are the farmers really doing? –Estimating local water management practices can be challenging at best

9. 9 Now on to Delta AG Water Quality