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

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.

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.

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.

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

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

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.

FAO 56 Penman-Monteith

Standardized ASCE Penman-Monteith

Hargreaves 1985

SCS modified Blaney-Criddle

Priestley-Taylor

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

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

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%)

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

ETc Map FAO 56 Penman-Monteith Method

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

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

References Li, Shujun, Tarboton, David, Mckee, Mac. "An ArcGIS Extension for Regional ET Estimation." ArcET. 18 April 2005. Utah Water Research Laboratory. 28 November 2005. <http://hydrology.neng.usu.edu/arcet/>.

Questions ????????????????????????????????????????????????????????????????????????????????????????????????