Here are some formulas you’ll need for drawing up nutrient management plans for out-of-ground container nurseries and greenhouses in Maryland. University of Maryland Extension experts have provided information on risk assessment of runoff from these operations. This information will help growers prepare information for a nutrient management plan or make changes in water management to improve their operations.
The management of water plays a key role in the nutrient management planning process, since nitrate nitrogen and orthophosphate are soluble.
The characteristics of the production site, the irrigation system and irrigation practices can be important contributors to the movement of nutrients from the area.
Runoff is of primary concern because it is the means by which water and nutrients move off the property into public surface water — streams, rivers and ultimately the Chesapeake Bay.
The assessment process involves evaluating the efficiency of irrigation practices to determine the amount of water applied and calculate the potential runoff.
Several terms are defined along with an explanation of how they are used in the risk-assessment process.
Leaching Fraction (LF) is a measure of the excess water that is applied during an irrigation event. (See Figure 1). It is the amount of water that runs out the bottom of the container divided by the total amount of water applied to the container. The goal is to manage water applications more accurately and reduce the leaching that contributes to runoff.
LF provides a means by which a grower can evaluate irrigation efficiency. Irrigation scheduling should be based on a measure of the amount of water needed to wet the substrate in a container without overwatering. A moisture sensor would be an alternative method for measuring water content. LF will vary over the life of a crop and irrigation scheduling (length of irrigation time) should vary accordingly. For both a nutrient management plan and in making management decisions, these evaluations should be made on a crop when a typical irrigation is required.
Interception Efficiency (IE) is a theoretical measure of the amount of applied water that is captured by the containers during an overhead irrigation event; the rest falls onto the ground around the containers. (See Figure 2.) IE describes the percentage of water going into the container, and 100 - IE is the rest, or that which falls directly onto the ground.
The amount is calculated by dividing the “container top area” by the “ground area allotted to one container.” The “container top area” is the open area of the top of the container through which irrigation water enters the container.
IE helps to evaluate the effect of container spacing on the distribution of irrigation water onto a crop. A large percentage of water (100-IE) misses the container for widely spaced containers and this contributes to runoff.
This value is particularly important in those operations that “fertigate,” for example, apply soluble nutrients in the irrigation water.
Figure 3 illustrates the calculation of IE. For a square container spacing, a rectangle drawn from center-to-center of four adjacent containers contains the top area of one container and the ground area allotted to one container. The center-to-center measurements of length down and row and width between rows is used to calculate ground area per container. The four quarters of the tops of a container sum to give one container top area.
Figure 4 shows the same calculation for diagonal container spacing. The width is now the perpendicular distance between rows. The area A1 within the parallelogram equals area A2. Moving area A1 to A2 creates a rectangle of length, L and width, W. There is one container top area within the parallelogram.
Runoff is the excess water that flows from the growing area being irrigated and moves toward offsite to surface water. For overhead irrigation, runoff is made up of water that leaches through containers plus the water that is not intercepted by containers. For drip or lowvolume irrigation, runoff is that proportion of the water that leaches through the container.
Amount of water applied and risk of runoff. Where that water goes determines if there is a runoff problem. Many factors may influence the length of time it takes to water a crop, including size of container, type of substrate, number of times water is applied daily, and the size and maturity of the crop. The primary interest is to improve the efficiency of irrigation, to reduce the risk of nutrient runoff to surface water.
The amount of irrigation water can be measured in several ways, but a leaching fraction test is one way to measure both the application amount and the amount of water that runs through a container. (See Figure 5.)
Line two empty containers with plastic bags. Set a container with a plant into one lined container, using a rock or other support so water can drain through the planted container. Set the second lined container near the first in the nursery or greenhouse. Run the irrigation system through a normal irrigation time period. Now, measure the amount of water in each lined container after the irrigation is completed, allowing the plant-occupied container time to drain completely (about an hour).
The leaching fraction is the amount of water that has drained through the plant-occupied container divided by the amount of water caught in the empty lined container. This fraction can be expressed as a percentage. A leaching fraction test should be done using several pairs of lined containers to ensure repeatable data is collected and a representative leaching fraction is determined.
Depth of Application. Determine the open area of the top of the container. Area is 0.785 times diameter times diameter. Diameter in inches can be used. Convert the volume of water in the empty lined container to gallons. Gallons can be converted to cubic inches by multiplying by 231. Depth of water applied is volume (in cubic inches) divided by container top area (in square inches) to give the depth of water applied (in inches). (See Figure 6.)
Total Applied Water. Determine the total growing area for the crop where the leaching faction was determined. Several areas may need to be tested to evaluate one watershed draining to a containment pond or area of the nursery. Multiply the total area (in square feet) times depth of water (in feet — converting inches to feet) to get cubic feet.
Potential Runoff. The potential runoff is estimated by the following equation using interception efficiency (IE) and leaching fraction (LF). Just plug in the numbers and get volume in cubic feet.
Potential Runoff = Total Applied Water x [(1.00 - IE) + (IE x LF)] where IE and LF are expressed in decimals, not percentage. (Division by 100 to convert from percentage to decimal is not shown here in the equation). (See Figure 7.) This potential runoff can be reduced to account for evaporation and infiltration, but it illustrates the potential maximum value.
Containment Basin. To qualify for low risk, the volume of the containment basin must hold at least 90 percent of the potential maximum daily runoff from the site plus some recycling of water from the basins or some provision for handling the overflow.
Calculate your containment pond or basin volume and compare. Recycling of the water could be a requirement to make room for the runoff of the next irrigation event. A treatment process using a buffer strip or a wetlands, diking to control flow or additional containment capacity downstream can reduce risk to surface water.
This illustrates the process to establish a value for amount of runoff water. Leaching fraction and interception efficiency, together with the fertilizer source, application rate and frequency all combine to determine the overall risk of nutrient runoff.
For more information, contact David S. Ross, Extension agricultural engineer, Department of Biological Resources Engineering, College Park; John D. Lea-Cox, nursery research and Extension specialist, Department of Natural Resource Sciences and Landscape Architecture, College Park; or K. Marc Teffeau, regional Extension specialist, Wye Research and Education Center, Queenstown.