Drought Management for Horticultural Crops
Larry A. Stein, Associate Professor and Extension Horticulturist
May 24, 1996

Horticultural crops demand and require significant amounts of water due to their perishable nature. Tree fruit and nut crops are not only comprised of large amounts of water, but the trees are perennial plants. Stress not only affects the current season's crop, but future crops as well. Vegetables are also quite perishable but they represent annual crops and thus only one year of production is affected.

Since water is such a critical component of the growth and development of horticultural crops, it is recommended that none of these crops be established without full irrigation capabilities. Normally 8-10 gallons of water per minute per acre well capacity for each acre planted are required for horticultural crops. Even with this level of water use requirements, there are several management decisions which can be undertaken to reduce the risk of over-extending an irrigation system.

Although drought management decisions are generally the same for vegetable and orchard crops, orchardists are looking at such steps to ease their water shortage immediately since they already have established, perennial trees. Vegetable producers have the ability to evaluate all aspects of their water situation prior to planting and thus have the opportunity to reduce the size of planting, not plant at all, etc. to make this water reach. Aspects to be considered on orchard crops include: current irrigation method inefficiencies, irrigation scheduling techniques, less than optimum irrigations, block productivity, and more efficient irrigation systems.

First and foremost though, the size of the operation must be adjusted to the capacity of the well or water source. Most producers try to stretch their operation past the limits of their water. In normal years when we get significant rainfall, we often get by with this deficit water budget. However, in times of drought, these shortfalls cannot be overcome. It is important that one pull back to what he can effectively irrigate, be it trees or vegetables. If not, serious crop damage could result to the entire crop rather than abandoning only a part of the crop.

Critically evaluate all soil types of your farming operation. Obtain a soil survey book and become aware of the limitations some soils have - also note the location of your best soils. Then plant accordingly. Some soils will never have the capability to provide an economic return. Avoid these areas and concentrate on your best soils. This could also mean that you will eliminate certain areas of established trees. Even established trees can be a liability if they are established on the poorer soils. Do not let site limitations cloud your economic intuition. It is far better to spend dollars on your best blocks where you have the best chance to make a crop than to pour dollars into very marginal areas.

Make a serious analysis of your current irritation system - everything from the efficiency of the pumping plant to the leaks in the distribution main lines or manifolds should be evaluated. It may be necessary for an outside expert to check out the system - as many times we fail to grade ourselves as critically as need be. It is not uncommon for system efficiency to be less than 50%.

Flood, furrow and/or basin are the most common irrigation methods practiced to date. They are excellent systems but often result in inefficient use of water due to overwatering beginning and ending segments and underwatering the middle segments. Depending on the system used and design, these systems have a 55-75% efficiency rating; can be better if properly designed. Many times inefficiency is made worse due to the failure to capture tailwater. Length of runs should be dictated by soil types and not field size. Laser planed pans can provide for very efficient use (potentially 95%) of water. Surge flow has been used successfully to reduce total water required and should be investigated. Furrow diking can also be used to capture rainfall between irrigations.

Flood irrigated orchards may consider watering every other row middle and/or furrows can be used to more efficiently water flood orchards and subsequently water every other furrow. Lay flat and gated pipe are far more efficient than an open ditch. Both are well adapted to a surge flow system which more effectively and efficiently waters flood runs. Open ditches have already been banned in some areas.

Sprinkler irrigation offers the ability to irrigate uneven terrain effectively. However, the energy required to pressurize the water can become expensive. System efficiency (ranging from 50-75%) can be reduced by poor nozzle sizing, evaporation losses and operational pressures. The new low energy precision application (LEPA) systems are increasing in popularity in vegetable applications. These systems have a very high initial cost but have tremendous efficiency. Few producers can afford to upgrade to improved irrigation systems during these drought stress times.

Drip irrigation has been used extensively by fruit and nut producers, although sparingly by vegetable operations. Drip can improve water use efficiency (80-95%) and total water use if the system is designed properly. Design criteria cannot be bypassed or ignored. Drip also represents a totally new way to irrigate crops, ie. prevent the crops from ever becoming stressed. However, drip does not reduce the overall plant water requirements. Drip is a method which must be learned and managed properly to get the most out of the system. Initially, there are increased labor costs for installation, but virtually maintenance free during the growing season. One must take the initiative to learn how to make it work.

Not only is irrigation scheduling critical to the success of drip irrigation, it can also be used to improve the overall system efficiency of the other methods as well. Knowing crop water use based on crop coefficients and/or canopy size, along with pan evaporation data, is an excellent way to estimate potential water use. This, along with the soil water holding capacity (obtained from the SCS books), allows one to schedule irrigations accordingly.

Still, it is important to monitor your system and determining what and where the water is wetting the soil. Tensiometers, gypsum blocks and water mark sensors are all good monitoring tools, but they are only as good as the manager.

Probably the best method is to combine these devices with a shovel and the actual "feel" of the soil, ie. if it makes a ball, it is wet.

In addition to drip, plastic mulch increases water use efficiency on vegetable crops. Plastic mulch reduces potential evaporation, the downward movement of water and weed growth. Organic mulches can also be used both in vegetables, as well as fruit operations. Such mulches also provide the potential for soil improvement due to the addition of organic matter. Organic matter increases soil water holding capacity.

Plastic mulch can be used to improve rainfall capture, thereby reducing pre-plant water requirements. These systems must be applied in early fall and must be used in cooperation with windbreaks to make them work.

Weeds are severe competitors for water and nutrients. It takes about 80 gallons of water to grow one pound of weeds - if the weeds don't get the water, the crop has a greater chance of getting it.

There are many ways to control weeds including: mechanical, herbicides, mowing and combinations of these practices. The method of choice today is probably some type of herbicide. Herbicides can do a good job but they don't work miracles. The ground must be in good shape, ie. clod free, they must be applied at the right rate and must be properly incorporated. Except for grasses, it is very hard to clean pre-existing conditions once the crop is up and growing.

Crop selection has a lot to do with drought management capabilities. If the water source is unreliable - naturally we should avoid establishing long term perennial crops. Let your water source, quantity and quality dictate what you plant.

Fruit and nut crops are perennial crops demanding water for the current crop, as well as crops in future years. Plan orchards where a long term reliable water source is available. Vegetables are annual crops, requiring large amounts of water to establish the crop and carry it to maturity. Crop selection should also be based on the reliability and the volume of water available. High water demanding crops include broccoli, peppers, potatoes and tomatoes. Crops less critical of exacting water delivery include greens, lettuce, spinach, cantaloupes and squash.

The midbed trench has been used successfully in many areas to improve the emergence and stand establishment of fragile seedlings like peppers and tomatoes. So land preparation can also affect how efficiently our plants harvest and use water.

Finally, if land is not limiting then one could move to extremely wide spacings on dryland plantings. This allows the potential crop to exploit a very large volume of soil in order to secure its water. In an irrigated situation, one would want to go to closer spacings to increase system efficiency.