1. Determination of Distributed Crop Water Requirements at Regional Levels using ArcET
By Josh S. Brown
CEE 6440

2. What is Evapotranspiration?
Evapotranspiration (ET) is the sum of evaporation and plant transpiration. Evaporation accounts for the movement of water to the air from sources such as the soil and waterbodies. Plant transpiration accounts for the movement of water within a plant and the loss of water as vapor through its leaves.

3. Why is ET important?
In areas that are not irrigated, ET is usually no greater than precipitation.
If a region’s ET is greater than its precipitation then soil will dry out unless irrigation is used.
Determining regional ET values allows for crop water requirements to be estimated.
Allows farmer to gauge the amount of water to put on crops so it is not wasted.

4. Methods for Calculating ET
FAO 56 Penman-Monteith
Standardized ASCE Penman-Monteith
Hargreaves 1985
SCS modified Blaney-Criddle
Priestley-Taylor
There are a few others methods for calculating ET, but ArcET only uses the five listed above.

5. ArcET Inputs 5 Main Data Inputs
Weather Stations – Location of weather stations.
Weather Data Table – Weather information at weather station (i.e. precipitation, wind speed, humidity, etc.)
Digital Elevation Model (DEM) – To get elevation data for the area of interest to interpolate weather information.
Land Use/Cover Shapefile – An ESRI polygon shapefile for crop distribution information
Crop Coefficients Table – A DBF table with crop coefficients for each crop type in region

6. Calculating Reference ET
Weather Stations
Weather Data Table
Digital Elevation Model (DEM)

7. Determining Regional ET with ArcET
Instead of only having 165 point- specific ET values (at each weather station), ArcET estimates the weather data for the whole region to get spatially distributed ET over the region. This is done by interpolating temperature, humidity, solar radiation, and wind speed data and apply it as necessary to the ET equations.

8. FAO 56 Penman-Monteith

9. Standardized ASCE Penman-Monteith

10. Hargreaves 1985

11. SCS modified Blaney-Criddle

12. Priestley-Taylor

13. Calculating Crop ET (ETc)
Land Use/Cover Shapefile
Crop Coefficients Table
ET Raster

14. Crop Distribution and Coefficients
Join Crop Coefficients (Kc) with Crop Distribution to get Crop Coefficient map

15. FYI Crop Types in Utah Alfalfa (50%) Beans (0.01%) Corn (0.05%)
Fruit (0.02%)
Grain (20%)
Grass (0.01%)
Pasture (30%)
Potatoes (0.01%)

16. Getting Crop ET ETc = Kc x ET ETc
Crop Coefficients Reference ET Crop ET

17. ETc Map
FAO 56 Penman-Monteith Method

18. ETc Results
There where 84,000 crops on the shapefile and only 1,300 ETc values

19. Conclusions
Hand calculations for selected areas gave similar results, usually within 0.5 mm/day
When comparing ET values for water users associations websites for certain areas, ET values where comparable, usually within 0.5 mm/day
Usually ArcET results gave results within 0.5 mm/day of each other, but had extreme differences as high as 3 mm/day

20. References
Li, Shujun, Tarboton, David, Mckee, Mac. "An ArcGIS Extension for Regional ET Estimation." ArcET. 18 April Utah Water Research Laboratory. 28 November <

21. Questions
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