Irrigation Overview

Commercial crops are routinely grown outside of the geographical regions to which they are have become adapted. In humid areas, the water demands for most crops are satisfied by rainfall. Crops grown in, but not native to, arid and/or semi-arid regions have water requirements that are generally larger than what seasonal rainfall can provide. For these crops additional water needs to be provided in the form of irrigation to produce yields or qualities that are economically viable. Irrigation has been around for thousands of years and takes many forms. Over time an irrigation science has evolved that deals with the study of soil-water-plant relationships to increase the efficiency and effectiveness of irrigation.

Improper irrigation can and does waste water, energy, and other resources. If more water is applied than needed then this extra water can carry nutrients, amendments and sediments from the site of the irrigation event. This can create problems down slope or down stream from the agricultural setting. Insufficient irrigations can and will impair the productivity of the soil which will decrease or eliminate plant productivity. Therefore, it is extremely important to know when to irrigate and how much water to apply during the irrigation event. This requires a basic knowledge of the soil-water-plant relationships.

Water enters and leaves this soil-water-plant relationship in a number of ways. Water enters this system through rainfall, irrigation, and capillary rise. Capillary rise is the process where water move upward through the soil profile from a ground water source. Water exits the system through runoff, deep percolation, and evapotranspiration (ET). Deep percolation is the process where water moves downward through the soil due to gravity and moves deeper than the crop roots extend into the soil. This water is no longer available to the plant for use.

Determining the amount of water entering and exiting this relationship is a very complex process. A weather station measures the rainfall amounts and calculates the ET value from a number of measured weather factors. The IMS program uses this information and the soil-water budget method to calculate the future water needs of crops grown in the District.

Irrigation water is delivered by a number of means in the District. This includes drip, microspray, portable, under tree, and overhead systems. The discharge rates (the amount of water leaving the irrigation equipment) varies from 0.05 inches per hour (in/hr) to over 0.3 in/hr. The efficiencies (the percentage of water the actually enters the soil profile) varies from 75% to 95%. This information is critical in determining how long to irrigate to replace the depleted water in the soil.

Take an apple orchard with overhead sprinklers and a vineyard with a drip system as examples. Apples can use approximately 3 inches of soil moisture before they need to be irrigated and overhead sprinklers deliver approximately 0.3 in/hr at 75% efficiency. The vineyard can use approximately 4 inches of water before irrigation is required and drip systems deliver 0.05 in/hr at 92% efficiency. The following calculations demonstrate how much time is required to run the irrigation systems to replace the needed water.

Apple: (3 inches)/[(0.3 in/hr) X 0.75] = 13.3 hours

Grape: (4 inches)/[(0.05 in/hr) X 0.92] = 87 hours

There is a huge difference in the amount of time required to replace the depleted water in these two examples, yet these are actual requirements for two sites in the IMS program.

Irrigation is not simply watering plants occasionally.  Proper irrigation requires the grower to have a working knowledge in botany (crop root zones, growth stages, and crop coefficients), soil science (type and depth of the soil), and fluid mechanics (sprinkler discharge rates and efficiencies).  With this expertise the farmer can get more "Crop per Drop."