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
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
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.