Watershed Modeling in areas with Intensive Agricultural Irrigation

1. Watershed Modeling in areas with Intensive Agricultural Irrigation Presented by: Jeremy Wyss, H.I.T Tetra Tech 25th Annual Alabama Water Resources Conference Orange Beach, Alabama

2. Ag Irrigation Extent and Importance • 2007 Census of Agriculture combined with the 2008 Farm and Ranch Irrigation Survey “provide one of the most complete and detailed profiles of irrigation in the United States” • 55,000,000 Irrigated Acres or 28% of farm land • 93% of irrigated land is Cropland • 60% by sprinkler and 40% by gravity • Estimated average of 1.7 acre-feet/acre (20”) of water application • 40% increased corn yield and 30% increased soybean yield • Climate models for the southeast project precipitation to come in less frequent, more intense events and also project temperature increases, thus decreasing soil moisture storage • Irrigation will become very important to augment soil moisture storage to sustain crop yields

3. Conceptual Hydrologic Impacts • Reduced Infiltration • Soil Crusting • Chemical and Physical • Increased soil moisture storage • Maximum infiltration reached sooner • Stream-flow Impacts • Reduction due to direct and indirect pumping • Peak flow and storm-flow increases due to reduced infiltration • Return flows and consumptive use are difficult to characterize and quantify

4. LSPC Watershed Model LSPC = Loading Simulation Program, C++ • Rainfall-runoff, lumped land use, pollutant loading simulation model • Streamlined Hydrologic Simulation Program FORTRAN (HSPF) algorithms for pervious and impervious land flow and pollutant transport • Potential for very large-scale modeling • A series of individual hydrologically connected sub-watersheds • Sub-Watershed • Weather Data • Land Use Distribution • Representative Soil Type • Reach • Weather Data • FTable • Reach Group

5. To Compute Deficit Time ET Days LSPC Simulated Irrigation Demand PREC & PEVT ) evaluated over… Irrigation Demand = f ( ETc (Crop Factor) If ET Days = 0, then Irrigation Demand= f (ETc * PEVT) Only PEVT* ETc - PRECIP

6. LSPC Irrigation Source Water • Surface Water • From a simulated Reach • Can be from any simulated Reach • Allows for “regional” irrigation withdrawals • Groundwater • New water to the model • Can not withdraw water from groundwater storage of the model…yet? *In the basic model structure you can not have water from two different sources being applied to the irrigated land within a modeled sub-watershed

7. 1. To PREC 2. To SURS 3. To UZS 4. To LZS 5. To AGWS LSPC Options for Irrigation Application Model Storage Irrigation Type Sprinkler Flood Buried Shallow Buried Deep Seepage

8. Ag Water Pumping Report UGA selectively monitored irrigators application amounts • Divided Georgia into four Reporting/Summary regions • Monthly Averaged irrigation depth by Source and Region • Min, Mean, Max values • Supplied Mean for normal years and Max for drought years

9. Irrigated Field Coverage A shape file, reflecting 2007 irrigated area, was created by the University of Georgia 27,275 Polygons (fields) Individual field acreage Individual field source water percent In theory…the impact of each individual field is represented in the model

10. Data Processing and Simulation Watershed B 50 acres Surface water 30 acres Groundwater Watershed A 47.5 acres Surface water 22.5acres Groundwater Irrigated Field 100 acres 75% Surface 25% Ground Irrigated Field Calculation 75 acres SW 25 acres GW A B Irrigated Field Calculation 20 acres SW 20 acres GW Irrigated Field 10 acres 25% Surface 75% Ground Irrigated Field 40 acres 50% Surface 50% Ground Irrigated Field Calculation 2.5 acres SW 7.5 acres GW A B Split Field B 40% Recalculated 30 acres SW 10 acres GW Split Field A 60% Recalculated 45 acres SW 15 acres GW Use acreage and regional mean irrigation depth to determinevolume of water from source acre-inch per month converted to cubic feet per second Water is irrigated back onto the land based on irrigation demand calculation If pond is empty then no irrigation occurs Withdrawals occur independently of irrigation demand Irrigated area became its own simulated land use and was removed from the original land use by determining the land use “under” the polygons Groundwater Groundwater

11. “Observed” vs. Simulated Irrigation

12. Scenario Layout Scenarios compared at USGS 02355350 – Ichawaynochaway Creek below Newton , Georgia • 1040 square miles • 160 square miles are irrigated (15% of area) • 54% Surface Water and 46% Groundwater Scenario 1 – No Application • Irrigation water pulled from surface sources but not applied back to the land • Analogous to treating irrigated water as a loss from the system Scenario 2 – No Irrigation • No water being pulled from surface sources • Analogous to ignoring irrigation

13. Scenario Results – Timeseries Scenario 1 Scenario 2

14. Scenario Results – Duration/Accumulation Scenario 1 Scenario 2

15. Scenario Results – Statistics • Lower volumes for Scenario 1 are expected (remove water from system) • Large differences in low flow simulation (low flow = drought) • Soil moisture storage causes lower peak flows and storm volumes Agricultural irrigation is not insignificant and just removing water from the system over predicts the hydrologic impact. Applying irrigation water back to the land is an important component of simulating Irrigation

16. Modeling Irrigation in areas with Intensive Agricultural Irrigation Comments/Questions?