Cotton Incorporated for financial support under research contract Project No. 17-555-NC.
North Carolina State Climate Office for development, implementation and hosting of the website.
Dr. Garry Grabow, Professor with the North Carolina State University Department of Biological and Agricultural Engineering, for providing data and advice on the development of the tool.
About This Tool
The Farm Water Needs Tool is primarily a planning tool to help users determine how much water in addition to rainfall is needed to fully satisfy crop water demand and thus realize full yield, and to provide estimates of required farm pond storage capacity for full irrigation under normal conditions. The tool is designed to provide:
1. Seasonal and monthly estimates of irrigation water demand for cotton and other field crops (peanuts, corn, and soybeans).
2. An estimate of approximate farm pond capacity required to fully irrigate selected crop.
3. An easy way to evaluate several scenarios by varying input data.
Required user input is location, crop, planting date, cropped area, area of the watershed draining to the proposed pond site, and the irrigation system type used.
Rainfall seldom fulfills the entire water needs of most agronomic crops in North Carolina due to periods of inadequate rainfall during the growing season. Irrigation helps to meet the deficit in crop water demand, but must be accompanied by a reliable water source. One source of irrigation water supply are “farm ponds” constructed in locations to capture runoff and located near cropped land intended to be irrigated. The required capacity of farm ponds is dependent upon rainfall runoff into the pond (water supply) and the irrigation water demand (water demand), and also the timing of the water supply and water demand.
Monthly estimates of rainfall from long-term weather data (30 years) are derived from the University of Idaho's Gridded Surface Meteorological Dataset (UofI METDATA), which has a 4km spatial resolution, while monthly estimates of runoff are derived from a water balance model called the “North American Land Data Assimilation System Phase 2” (NLDAS-2) Noah surface model that uses long-term weather data (30 years) to partition rainfall into evapotranspiration (ET) and runoff. The output of this model is provided at a ~12km grid resolution. Monthly crop water demand estimates are obtained from a reference ET gridded dataset that is multiplied by a crop coefficient (to adjust for crop stage). Monthly irrigation water requirements are then computed by subtracting the expected contribution of rainfall (effective precipitation) falling directly on the cropped fields from the crop water requirement.
Output is provided in tabular form that shows long-term monthly rainfall and runoff at the selected location in inches, runoff in acre-feet (1 acre-foot is equal to 12-acre inches or 325,848 gallons) that is dependent on the user-input watershed area, irrigation water demand in inches and acre-feet, the monthly deficit that must be met by pond storage, and cumulative runoff and irrigation water demand. The required capacity (in acre-feet) of a farm pond to meet full irrigation water demand is provided at the bottom of the table. If runoff is insufficient, a note to that effect is shown instead of the required storage volume.
The tool is useful even if farm pond construction is not planned as the table provides long-term irrigation water requirements that are helpful for irrigation water management purposes.
A hypothetical example is shown below for cotton planted near Scotland Neck, NC. The proposed farm pond location has a contributing watershed area of 1200 acres to supply water for irrigation of 200 acres of cotton. In this example the planting date is May 15.
The user may simply navigate the map by dragging the screen and zooming (ctrl + scroll) and then clicking at the proposed location of the farm pond. A marker will be placed at the location and the latitude and longitude will display in the box at the top of the screen. Alternatively, the user can enter a location in the box as a search and a marker will appear at the searched location. The location of the marker can be refined by further clicking.
Map Navigation and Location Selection Video
Drop down boxes below the map are used to enter the required information. The "submit" button is then clicked to produce output.
Input Selection and Watershed Area Determination Video
Figure 2 shows tabular output produced by the tool. In this example, the long-term irrigation water requirement is 15.9 in. and the estimated required storage is 130 acre-feet. Note that despite the estimated long-term annual runoff into the site is greater than the average long-term irrigation water demand for this scenario (cumulative annual values of 297 and 265 acre-feet, respectively, in this example), storage is still required due to the different timing of runoff and demand. In this scenario, there are 4 months with a water deficit that must be met by carry-over storage from previous months.
Table Interpretation Video
Figure 3 shows the long-term expected monthly rainfall and runoff (note separate y-axis for each). A greater proportion of rainfall converts to runoff in the winter months (greater relative separation of rainfall and runoff lines in Figure 3) when evapotranspiration in the watershed is low. Figure 4 plots the last 2 columns of Figure 2 that show the relative rates of accumulation of runoff and irrigation water demand. Note that while in the web-interface, an icon with 3 small horizontal lines appears to the upper-right of both graphs and may be clicked to download or print the graph (chart). Figure 4 shows the greatest surplus in cumulative runoff in June and a slight cumulative water deficit by October. Placing the cursor on the points of the plots produces the plot values that correspond to the values in the table of Figure 2. If cumulative runoff does not exceed cumulative demand, no amount of pond storage capacity will result in full irrigation. In this case, on average, the irrigated acreage will not be fully irrigated. Options for this case include 1.) partial irrigation 2.) reducing acreage for full irrigation of the reduced acreage, or relocating the pond “downstream” or at another location that has a greater contributing watershed area.
Graph Interpretation Video
One of the helpful features of the tool is the ability to rapidly run different scenarios. Once a location has been selected, the user can change the crop and irrigated acreage to see the impact on irrigation water requirements and required pond storage. A user may typical want to know how many acres can be irrigated at the location selected. With the location and crop selected, the user can vary the irrigated acreage to see the effect it has on required pond storage. Irrigated acreage can be increased until a message “surface water supply inadequate…….” appears at the bottom of the output table, which can be interpreted as an expectation that during a normal year with the irrigated acreage entered for the selected crop, the water supply will be inadequate for full irrigation no matter the size of the farm pond. Changing irrigate acreage incrementally will also give the user an idea of the change in pond capacity required for the additional incremental fully irrigated acreage. The relationship is not necessary linear so the tool helps quickly assess the trade-offs – increased irrigated acreage (benefit) for a given increase in farm pond capacity (cost).
Different Scenario Output Video
This tool is meant to be a planning tool, and thus uses long-term averages of rainfall and climate to derive estimates of runoff, crop water demand and associated estimated required farm pond size based on an “average” year. It will not serve to determine water requirement for any particular year or current year, so is not meant to be used for irrigation scheduling.
This tool is also limited to estimation of long-term average irrigation water requirements of cotton, peanut, soybeans, and field corn, and thus is not suitable for use with other crops, such as vegetable crops, orchard crops, or vineyards, etc. Also, only one crop can be selected for any given scenario, rather than a mix of crops with their associated acreages. Due to crop timing and water requirement interdependency, adding required storage for one crop scenario to the required storage for another crop run independently in another scenario will not equate to the required storage of the intended crop mix. Rather, it is suggested to run one scenario using the crop with the greatest water demand.
The tool will not assist in the determination of watershed area draining to the farm pond location selected on the map interface. There are other tools and resources to help assist in watershed area delineation – these can be found below in the “Additional Resources” section.
The tool is likely to produce output that is on the conservative side, i.e., the estimated required storage may be more than needed, based on a conservative estimate (low) of long-term average runoff, and a conservative estimate (high) of long-term average irrigation water requirement.
In addition, this tool and its output should be used with extreme caution in the mountains of North Carolina since the runoff estimates provided are much lower than expected. A caution message will appear at the top of the screen when you have selected a location in the mountains, which reiterates the need to use caution when interpreting the results.
Finally, this tool has only been evaluated for North Carolina and therefore will not return any output if a location is chosen in a different state. If this occurs, please go back and select a location in North Carolina.
General USGS Quadrangle Information Site:
- Can be used to help delineate pond watershed and determine watershed area.
- The map viewer may also be used, from which one can search or zoom in to desired area. PDF Files of topographic maps with layers may be downloaded.
NRCS Local Service Area Offices for North Carolina:
- May be used for technical support.
North Carolina Division of Soil and Water Conservation - Technical Services:
Tool developed by Garry Grabow, Veronica Fall, Aaron Sims, and Heather Dinon Aldridge