MODELING IRRIGATION DEMAND IN RESPONSE

Transcription

1 Biofuels and Climate Change: Farmers Land Use Decisions (BACC:FLUD) MODELING IRRIGATION DEMAND IN RESPONSE TO LAND USE & CLIMATE CHANGE

2 IMPORTANCE OF THE HIGH PLAINS AQUIFER The High Plains Aquifer is one of the most important sources of fresh water in Kansas and the United States. 7% of the water used in Kansas every day comes from the High Plains Aquifer

3 IMPORTANCE OF THE HIGH PLAINS AQUIFER The ability to predict irrigation demand would enable an estimate of the future impact to groundwater levels based on varying climate or land use scenarios.

4 IMPORTANCE OF THE HIGH PLAINS AQUIFER Average annual recharge is only 1.5 million acre-feet. In 27, 4.4 million acre-feet were withdrawn. Various estimates of the aquifer s life vary by location. Some parts of the aquifer have less than 25 years of water left, while others have over 25.

5 PURPOSE OF THE PROJECT Two main goals: Develop a method for predicting irrigation use in the High Plains region of Kansas as a function of weather and crop choice Examine the link between withdrawals and changes in the water table Irrigation makes up the majority of groundwater withdrawals from the High Plains Aquifer. The ability to predict irrigation demand would provide an invaluable tool in predicting the impact of changes in climate and land use to the aquifer.

6 LOCATION OF STUDY The target counties were Barton, Finney, Morton, Reno, Stafford, Stevens, and Thomas. Weather was obtained from HPRCC stations (red squares) and the centennial stations (blue triangles). Irrigated acreages for each county were retrieved from NASS ; historic water use obtained from WIMAS.

7 THE WATER BUDGET MODEL A water budget keeps track of the amount of water in a system by tracking the inputs and outputs. ΔST = P + I ET R This can be applied to an irrigated field in order to track the soil moisture to determine the irrigation water needed. Inputs: Irrigation and precipitation Withdrawals: Evapotranspiration and runoff. Precipitation, Irrigation Evapotranspiration Runoff This calculation is performed on a daily timestep. Change in storage

8 THE WATER BUDGET MODEL We are using a modified version of Johan Feddema s water budget model to estimate irrigation in seven counties for 25 years. Additions to the model: Hargreaves-Samani ET, GDD based crop coefficients, irrigation scheduling based on soil moisture Model Needs: location information and daily weather data It needs daily maximum and minimum temps and precipitation It calculates actual evapotranspiration based on potential ET and soil moisture conditions. It keeps a running total of the soil water available for use.

9 Penman-Monteith is the standard for calculating evapotranspiration. EVAPOTRANSPIRATION CALCULATIONS Temperature, relative humidity, wind speed, and solar radiation are all required for Penman- Monteith but are only available at a limited number of sites in Kansas. Several ET methods were compared Average daily ET by month at Colby, KS, 2

10 EVAPOTRANSPIRATION CALCULATIONS The Hargreaves-Samani ET method requires fewer inputs than Penman Montieth, allowing for use at many more locations than the several HPRCC stations overlying the High Plains Aquifer. Hargreaves-Samani is also commonly used in Irrigation and Ag. Engineering. Hargreaves - Samani ET (mm) Hargreaves - Samani ET (mm) Colby R² = Penman Monteith ET (mm) Hays R² = Penman Monteith ET (mm) Hargreaves - Samani ET (mm) Hargreaves - Samani ET (mm) Garden City R² = Penman Monteith ET (mm) Manhattan R² = Penman Monteith ET (mm)

11 CROP COEFFICIENT ALL CROPS Reference ET for the water budget model will be calculated using the Hargreaves-Samani method Kc Corn Kc Grain Sorghum The reference ET is scaled using GDD-based crop coefficients Growing Degree Days ( C-d) Soybeans Growing Degree Days ( C-d) Winter Wheat K c curves were taken from research out of Bushland, TX and scaled based on KS recommended GDD maximums Kc Growing Degree Days ( C-d) Kc Growing Degree Days ( C-d)

12 IRRIGATION IN THOMAS COUNTY Required depth of irrigation water is determined (i.e., mm). This depth is multiplied by the irrigated acreage to get the irrigation need for that crop. The irrigation need for each crop is summed at the countyscale and compared to the reported water use in WIMAS. Thomas County s predicted and actual authorized irrigation withdrawals are shown at right. Year Area Planted in Corn (Acres) Irrigation for Corn (mm) Irrigation for Corn(acre-ft) Total Irrigation (acre-ft) , ,675 72, , ,822 92, , , , , ,276 11, , ,81 53,5 Irrigation Water (thousand acre-feet) Irrigation Water Required Irrigation Water Used

13 IRRIGATION NEED BASED ON MOISTURE DEFICIT At first, irrigation was not modeled in the water budget. We assumed that the irrigation demand would equal the moisture deficit at the end of the growing season. If the predicted water use exceeded the total amount of authorized withdrawals, then it was capped at that maximum amount. Good correlation, but slope >1. Predicted Use 6, 5, 4, 3, 2, 1, y = 1.343x R² =.9372 Barton Finney Morton Reno Stafford Stevens Thomas 1, 2, 3, 4, 5, Reported Use

14 MODELING IRRIGATION IN THE WATER BUDGET MODEL Recommendations for irrigation scheduling were taken from K- State s Mobile Irrigation Lab ( su.edu/mil/ ToolKit.htm) When soil moisture content falls below 5% (1 mm), then irrigation begins. Irrigation water is applied based on a theoretical maximum pumping rate of 8 gpm to a 125 acre field, or 12 mm/day. Irrigation continues until the soil moisture reaches 75% saturation (15 mm)

15 IRRIGATION DEMAND FROM SOIL MOISTURE The second analysis modeled irrigation in the water budget based on daily soil moisture conditions. The predicted use matches much closer to the reported use with this method slope of 1.2 instead of 1.3. Predicted Use 5, 4, 3, 2, 1, y = 1.154x R² =.96 1, 2, 3, 4, 5, Reported Use Barton Finney Morton Reno Stafford Stevens Thomas

16 ACCURACY OF THE WATER BUDGET MODEL Water holding capacity is a single value for each county a smaller scale will be more sensitive to local variation. Water holding capacity is for the top 1 cm of soil. This could be improved by altering the depth of the soil column based on different crops or crop stages. Irrigated acreage data from NASS is not perfectly accurate. 2 Predicted irrigation need / Reported Use Finney County Predicted irrigation need / Reported Use Thomas County

17 ACCURACY OF THE WATER BUDGET MODEL There is additionally uncertainty in the WIMAS database Water meters were not required until the 199s The database goes through quality control with Kansas DWR, but only since 199. The error between the model s prediction and the reported water use did not change as water meters were phased in throughout the state. Predicted Irrigation Demand / Reported Water Use Barton Finney Morton 1 Reno Stafford.5 Stevens Thomas Reported Use

18 ACCURACY OF THE WATER BUDGET MODEL Even if all the data used in the model were accurate, there are still sources for error between the model and reality. There are several complexities to Kansas water law, and water policy is known to restrict farmers water use in several locations. The Walnut Creek IGUCA is currently the only IGUCA that has restricted withdrawals, but most of the other IGUCAs are closed to new water rights. Some farmers have temporary stays to their water use, if they are in an area affected by minimum desirable streamflows For the last two months, we have worked intensely with a law student to understand and gather records on legal restrictions to water use.

19 CONCLUSIONS The use of a water budget model allows for the prediction of irrigation water use, based on land use and climate. This calculated irrigation demand was compared to historic reported water use, with a slope of 1.15 at the county-scale. Initial runs of the water budget with a simple climate change scenario indicate several refinements that may need to be made.

20 Biofuels and Climate Change: Farmers Land Use Decisions (BACC:FLUD) MODELING IRRIGATION DEMAND IN RESPONSE TO LAND USE & CLIMATE CHANGE

21 IMPORTANCE OF THE HIGH PLAINS AQUIFER The High Plains Aquifer is an important source of fresh water in 8 states: Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming. This aquifer, however, is threatened: large portions of the aquifer have lost significant amounts of water.

22 IMPORTANCE OF THE HIGH PLAINS AQUIFER In Kansas, the principal aquifers are the Ogallala, the Great Bend Prairie, and the Equus Beds aquifers. These aquifers occupy ~3,5 square miles of Kansas, all in the central and western portions of the state.

23 DATA SOURCES KGS maintains databases of historic reported water use (WIMAS) and historic measurements of water levels (WIZARD). Irrigated acreage for each county was retrieved from NASS (yearly) and the Census of Agriculture (every 5 years). NASS is not complete some data are missing, as evidenced by Thomas County. Even when data exists, there in both datasets, there is not 1% agreement. Irrigated area is taken from NASS, and supplemented by the Ag. Census if necessary. Corn (NASS) Corn (Ag. Census) Sorghum (NASS) ,5 2,9 Sorghum (Ag. Census) ,3 7,182 2,1 1, ,2 1, ,4 1,4 2 81, ,2 3, ,8 59,522 NA ,5 8,1

24 GROUNDWATER LEVELS The yearly average water surface elevation was plotted for each of the target counties. This was plotted against the groundwater withdrawals for each county. Both Stevens and Finney Counties exhibit a water table that is dropping almost linearly while withdrawals stay more or less level. Irrigation Water (thousand acre-feet) Irrigation Water (thousand acre-feet) Finney County Irrigation Water Used Water Surface Elevation Stevens County Irrigation Water Used Water Surface Elevation Average Water Surface Elevation (ft) Average Water Surface Elevation (ft)

25 GROUNDWATER LEVELS BARTON COUNTY The general trend in the water table is up, not down. The withdrawals are an order of magnitude less than Finney and Stevens. In 1992, both the water surface elevation and the withdrawals both change dramatically the Walnut Creek IGUCA goes into effect. Irrigation Water (thousand acre-feet) Irrigation Water Used Barton County 1992: Walnut Creek IGUCA is established, drastically cutting groundwater withdrawals. Water Surface Elevation Average Water Surface Elevation (ft)

26 GROUNDWATER LEVELS STAFFORD COUNTY Like Barton County, Stafford County s annual water use and water table are extremely variable. Water use peaks several times, with each peak associated with a drop in the water table. This is seen clearly in 1988, 1991, 1994, 23, and 28. Irrigation Water (thousand acre-feet) Stafford County Irrigation Water Used Water Surface Elevation Average Water Surface Elevation (ft)

27 GROUNDWATER LEVELS MORTON COUNTY Morton County uses more groundwater annually than Barton, but much less than Finney or Stevens. For the first half of the time period, the water table is clearly in decline. From 1996 onward, the water table stays roughly level. There are no peaks and valleys in this plot that are as dramatic in the other counties, but there are several points where the water table drops 5 in one year only to gain it back the next. Irrigation Water (thousand acre-feet) Morton County Irrigation Water Used Water Surface Elevation Average Water Surface Elevation (ft)

28 GROUNDWATER LEVELS The four counties with the highest water use experienced declines in the water table, while the 3 with the lowest experienced rises or no change over the time period. Barton, Reno, and Stafford Counties overlie the eastern part of the aquifer, while the others are in the west. The seven target counties fall into three general categories of water use. County Average annual irrigation use (acre-feet) Average annual change in water surface elevation (ft) Finney 34, ± 59, ± 1.89 Stevens 25, ± 33, ± 2.28 Thomas 11, ± 23, -.53 ± 1.85 Morton 88, ± 25, -.83 ± 3.69 Stafford 83, ± 17,.21 ± 2.19 Reno 37, ± 11,.2 ± 1.38 Barton 34, ± 8,.41 ± 1.79

29 GROUNDWATER LEVELS Stafford Counties, on the low end of irrigation withdrawals, has water tables that is responsive to a change in irrigation use. Barton County is similar has a similar relationship. Morton County, on the other hand, has almost no link between the quantity of water withdrawn and the change in water table elevation. Finney, adjacent to Morton County, has an even lower r 2 (.11) Change in Water Surface Elevation, from year x-1 to x (ft) Change in Water Surface Elevation, from year x-1 to x (ft) y = -.1x R² = , 4, 6, 8, 1, 12, y = -2E-5x R² =.158 Total Irrigation Withdrawals, year x-1 (acre-ft) Stafford County 5, 1, 15, Total Irrigation Withdrawals, year x-1 (acre-ft) Morton County

30 CONCLUSIONS The relationship between withdrawals and drawdown of the water table needs to be further investigated. A qualitative analysis of the yearly groundwater levels for the seven target counties revealed that the water table is sensitive to variations in water use in some locations. The counties with high water use tended to have a water table that was constantly in decline. On the other hand, counties with lower water use tended to have a water table that is much more responsive to changes in water use. Change in Water Surface Elevation, from year x-1 to x (ft) Change in Water Surface Elevation, from year x-1 to x (ft) Barton County y = -1E-6x R² =.11 1, 2, 3, 4, 5, Total Irrigation Withdrawals, year x-1 (acre-ft) y = -.2x R² = , 2, 3, 4, 5, 6, Total Irrigation Withdrawals, year x-1 (acre-ft) Finney County