Irrigation Depths - Details and Data

Irrigation amounts were computed and summarized statistically to represent monthly applications that would be needed to meet normal crop water needs in wet, average, and dry years. For each major crop type, irrigation schedules and monthly totals were computed for weather conditions during each of the years from 1950 through 2007. These represent some extreme droughts as well as some years where little or no irrigation would have been used. After computing irrigation amounts for each month of all years, we ranked the results and then selected the 25th percentile (only 25% of the years had lower irrigation for that month and crop) to represent the wet year irrigation. Similarly we selected the median (half of the years had less, half had more) to represent normal years. For dry years we selected the 75th percentile (only 25% of the years had more irrigation). In tabular support data for each county, we also provide the 10th and 90th percentiles. They represent extremes of wet- and dry-year irrigation.
In examining values presented for wet, average, and dry years it is important to recognize that these represent means of irrigation that could be expected by farmers in that county. Individual fields may receive more or less irrigation depending upon timing of rain received at the field, farmer's yield goals, costs for setting up and applying an irrigation, availability of water from source supplies like ponds and streams, and other factors.
In recognition of observated irrigation patterns for major crops, we reduced irrigation demand for farmers using only surface water for their irrigation of cotton, peanut, corn, soybean, and pecan. These farmers are limited by water available in streams or rainfall that refills their ponds. In dry periods when irrigation is needed, these supplies are often inadequate. Typically, and in our computations, surface irrigators use 70% as much as those using wells. This was observed on 800 fields monitored each month for 5 years between 1999 and 2004 (Ag Water Pumping study). Recent dat from the states water metering program suggest likewise - farmers using surface water can't or don't apply as much water as those with dependable groundwater supplies.
For specialty crops, irrigation application depths could not be predicted by crop models. Although models have been built for many of the vegetable crops, and generic models can be used for other crops, most have never been validated in Georgia or the Southeast. Instead of relying upon models, we turned to the Georgia Irrigation Surveys for information on these crops. This was the same source as used for baseline acres of the specialty crops. County agents report average annual application depths used during the survey year. Thus for tobacco, peach, apple, blueberry, grapes, and sod, agents provide crop specific estimates of water use. For vegetables, nursery, and greenhouse, there are too many types of crops possible, so they report only four lumped estimates - vegetables produced under sprinkler (snap beans, sweet corn, onion, e.g.); vegetables produced under drip (staked tomatoes, peppers, egg plant, broccoli, e.g.), nurseries (both in-ground and container grown annuals and perrenials); and greenhouses (average water depth per covered area). While the survey years - 1992, 1995, 1998, 2000, 2004, and 2008 - do not represent as wide a range of conditions in weather as crop models, there was a reasonable spread of wet and dry years in those years. To create wet, median, and dry-year values, we lumped all reported depths for each crop over all years, and computed the statistics for the entire WPR. Thus all counties in the Lower Flint - Ochlockonee WPR, for example, have the same wet, median, and dry irrigation amounts.
The analysis used for major crops examines withdrawals by month for February through November. This analysis better describes months during the year withdrawals will be needed for irrigation. Irrigation Surveys do not report monthly irrigation amounts. To create this analysis for specialty crops, we turned to Ag Water Pumping study where monthly observations of irrigation application depths were observed. While there were fewer years included in the AWP data (5 to 6y) as compared with Hoogenboom (58y) predictions for major crops, and there were generally few or no observations in many counties (and some WPR had none for many specialty crops whether they were grown there or not), the pattern of monthly irrigation that was created in the 5 years is likely to reasonably represent when farmers in other counties, regions, and years. Using crop by crop statistics from the AWP study (data below), i.e. blueberry (2/y), grapes (1/y), peach (1/y), tobacco (29/y), sod (21/y), and nursery (5/y), ratios of month to annual application were calculated. Fresh and Process vegetables were made from weighted averages of annual averages from cole crops (9/y), cucurbits (12/y), melons (18/y), mixed vegetables (13/y), greens (2/y), onion (19/y), snap beans (13/y), solanaceous (4/y), and sweet corn (15/y). The numbers in parenthesis are number of fields being observed for each of the the 4 to 6 years of the AWP study. The ratios of monthly irrigation application depth to annual application (from)AWP were multiplied by the regional irrigation application percentiles for each specialty crop.
Further Details.
For more complete explanations of methodology and assumptions underlying crop model calculation of irrigation amounts for the major crops.

Data Sets:
  • Weather stations used
    County-by-county record of weather stations used in the models. (Excel file - last update 3/31/2009)

  • Georgia's Irrigated Soils
    Record of soils in mapped irrigated fields and soil series used to represent them in the models. (Excel file - last update 3/31/2009)

  • Predicted Irrigation Depths for corn, cotton, peanut, soybean, & pecan
    1950 to 2007 model-predicted irrigation depths by County and Soil. Data used for irrigation depths of major crops. (Excel files - last update 3/29/2009)

  • CES Surveys of Irrigation Depths
    1992 to 2008 survey data from CES Agricultural Agents on irrigation used by crop type. Data used for irrigation depths of specialty crops. (Excel file - last update 7/16/2009)

  • Specialty crop distribution of monthly irrigation.
    1999 to 2004 observed data from Ag Water Pumping used to create monthly proportions of annual irrigation. Data used for irrigation depths of specialty crops. (Excel file - last update 7/16/2009)


Last updated 7/30/2009 James E. Hook