Managing Irrigation to Control Salinity

Careful monitoring, drainage, and timely leaching help manage salinity on irrigated land.

Irrigation water, even high quality ground and surface water, carries with it dissolved salts. As the water is removed from the soil through the processes of evaporation and plant transpiration, the salts are left behind and begin to accumulate in the zones of water removal. Unfortunately, the zones of water removal are at the soil surface where seeds and seedlings must deal with them, and in the root zone where larger plants must overcome the attraction of water to the dissolved salts in order to remove water for growth. Eventually, even with high quality water sources, irrigation without regard to salinity management will result in the build up of salts to levels that will reduce plant growth.

Where salts have accumulated in soils to levels that begin to reduce plant growth, the only means of plant growth, the only means of correcting the problem is through the establishment of adequate leaching and/or drainage. The addition of chemical amendments, soil conditioners, or fertilizers will not only be ineffective in offsetting plant growth reductions due to salinity, they could exacerbate the problem by adding more salts to the root zone. Conceptually, the processes of leaching and drainage are nothing more than rinsing the dissolved salts out and below and/or away from the root zone of the plant.

There are several ways in which the removal of salts can be accomplished. The various methods can be grouped into three main categories. First, salts can be moved well below the root zone by adding extra water applied above the needs of the plant. This method is referred to as the leaching requirement method. The second method, here soil moisture conditions dictate, couples the leaching requirement method with artificial drainage to facilitate the removal of salts from the soil. Third, salts can be moved away from the root zone to locations in the soil, other than below the root zone, where they are not harmful. In this article we will refer to this third method as managed accumulation. Each of these options will be further discussed below.

Leaching Requirement
The leaching requirement method has been well documented and researched for the last half century. This method is a mass balance of salts in the soil where one attempts to match additions of salt in irrigation water by leaching the same amount of salt from the bottom of the root zone (salt in = salt out) thereby preventing harmful accumulation of salts.

The amount of salt in any water is the concentration of salt multiplied by the volume of water. Therefore, if we let "C" and "V" designate concentration and volume, and "i"and "l"designate irrigation water and leachate, we can write a simple salt balance equation as follows:

C_i x V_i = C_l x V_l
(or, salt in = salt out)

We can rearrange the equation so that we get:

V_l / V_i = C_i / C_l

It should be noted here that the concentration of salt in water is related to the electrical conductivity (EC) of the water as follows:

C (meq/l) = 10 x EC
(dS/m)

Published salinity tolerance limits for plants are generally expressed in terms of electrical conductivity, so for simplicity we can rewrite the second equation as follows:

V_l / V_i = EC_i / EC_l

In the above equation, the ratio of the volume of leachate to irrigation water is the leaching requirement. If one measures the EC of their irrigation water and chooses a value of the EC of the leachate, the leaching requirement can be calculated. Because salt should be controlled to levels that are not harmful to plant growth, the EC of the leachate should be chosen as the published limit for the plant of interest (see Table 4).

The use of a leaching fraction requires a couple key considerations. The leaching of salts can only occur if the soil is adequately drained. In other words, there should be no shallow water table to prevent the downward movement of the leachate, and the soil should be permeable enough to allow the extra water to flow through the profile without having to greatly increase irrigation set times or saturate the soil for long periods of time. Long irrigation set times may result in an inability of the grower to keep up with irrigation needs in other parts of the field and may cause excessive runoff. Long periods of saturation may result in aeration problems for the plant.

Additionally, if the application of irrigation water is not uniform, proper leaching will not be attained, and even higher, faster accumulation of salt may occur in the areas of the field that receive lower application amounts. Localized salinity problems may occur in fields that have poor water distribution and/or low-lying areas in the field where excess water from surrounding areas drains to. Therefore, the importance of irrigation uniformity can not be overstated.

For most surface irrigation systems in Colorado (furrow and flood) irrigation inefficiency is generally adequate to satisfy the leaching requirement. Surface irrigators should compare leaching requirement values to measurements of irrigation efficiency to determine if this is true for their operations. Adding more water to satisfy a leaching requirement will only further reduce irrigation efficiency and may result in the loss of nutrients, pesticides, and soil.

Leaching can be done on a limited basis at key times during the growing season, particularly when a grower may have water of high quality available. Surface water in most areas of the state tends to have lower salinity than shallow, alluvial groundwater. Deep groundwater may also be of high quality and can be of lower salinity than either shallow groundwater or surface water. In situations where a grower may have multiple water sources of varying quality, planned leaching events at key salinity stress periods for a given crop may be considered. Most crops are highly sensitive to salinity stress in the germination and seedling stages. Once the crop has grown past these stages it can often tolerate, and grow well in higher salinity conditions. Planned periodic leaching events might include a large, post-harvest application to push salts below the root zone to prepare the soil (especially the seedbed/surface zone) for the following spring. Fall is the best time for a large, planned leaching event, because nutrients have been drawn down that at other times during the season would move with leaching water and be lost. Additionally, in most years the soil water contents have been drawn down providing the most control over leaching salts to desired depths without pushing them further into shallow groundwater where they may become contaminants. As can be seen, each case is individual and all the soil, groundwater, drainage and irrigation system conditions for a given field should be considered in developing a sound leaching plan.

Leaching plus Drainage
Where shallow water tables would otherwise limit the use of the leaching requirement as discussed above, artificial drainage may be employed. Drainage ditches can be cut in fields below the water table level to channel away drainage water and allow the leaching of salts. Tile or plastic drain pipe can also be buried in fields in a drainage collection network. Proper design and construction of a drainage system is critical and should be performed by a trained professional. Consultation with local NRCS, or Extension agricultural engineering personnel will provide ideas and direction on proceeding properly with drainage system design.

With all artificial drainage situations, consideration must be given to the disposal of the drainage water. Some restrictions on the discharge of drain water to streams may apply in certain situations and should be investigated with the appropriate agency. In the case of regulated discharge, treatment or collection and evaporation of the water on site may be required and may add significant costs to the use of artificial drainage.

Artificial drainage provides the advantage of being able to use high quality, low salinity irrigation water (if available to a grower) to completely remove salts from the soil. It should be noted here that artificial drainage systems will not work where there is no saturated condition in the soil. Water will not collect in a drain if the soil around
it is not saturated.

After drainage appears adequate, the leaching process can begin. Table 5 gives a rough rule-of-thumb for how much water is required to leach salts. The actual salt reduction will depend upon water quality, soil texture and drainage.

For example, if a soil's electrical conductivity is 8 mmhos/cm, and we want to reduce electrical conductivity down to 4 mmhos/cm, this represents a 50% reduction in salts. Therefore, 6 inches of water would be required.

Double-row Beds

Good uniformity, salts accumulate in the center of the bed and away from plants.

Poor uniformity, salts accumulate toward edge of bed near one row.

Figure 1. Typical salt accumulation pattern in double-row beds.

Managed Accumulation
In addition to leaching salt below the root zone, salts can also be moved to areas away from the primary root zone under certain crop bedding and surface irrigation system configurations. Several examples of managing salt accumulation in this manner are illustrated in Figures 1 and 2. The basic idea is to ensure that the zones of salt accumulation stay away from germinating seeds and plant roots. In all of the configurations in Figures 1 and 2, irrigation uniformity is imperative. Without uniform distribution of water, the salts will build up in areas where the germinating seeds and seedling plants will experience growth reduction and possibly death.

Double-row bed systems require uniform wetting toward the middle of the bed. This leaves the sides and shoulders of the bed relatively free from injurious levels of salinity. Without uniform applications of water (one furrow receiving more or less than another) salts will accumulate closer to one side of the bed where a seedrow would be. Periodic leaching of salts down from the soil surface and below the root zone may still be required to ensure that beds are not eventually salted out.

Single-row Beds

Uniform, healthy plants with alternate furrow irrigation (salt accumulates in dry furrows)

Irregular growth due to variable accumulation of salt (plants may overcome this situation if roots can grow out of saline area)

Figure 2. Typical salt accumulation patter in alternate furrow and every furrow irrigation regimes.

Alternate furrow irrigation may be desirable for single-row bed systems. This is accomplished by irrigating every other furrow and leaving alternating furrows dry. Salts will be pushed across the bed from the irrigated side toward the dry furrow, accumulating there. Care must be taken to ensure that enough water is applied to wet all the way across the bed so that salts will not build up in the planted area. This method of salinity management may result in plant injury in cases where large rainfall events fill the normally dry furrows and push salts back across the bed toward the plants. This same phenomenon will occur if the normally dry furrows are ever accidentally irrigated.

Sprinkler Irrigation
Sprinkler irrigated fields where irrigation water quality is poor present a challenge because it is often difficult to apply enough water to leach the salts and you cannot effectively exploit row or bed configurations to manage accumulation. Growers need to monitor the soil EC and irrigation water salinity where water quality is poor. In some cases, the only viable management option is to plant salt-tolerant crops. Sensitive crops, such as pinto beans cannot be managed profitably in saline soils. Where adequate irrigation water exists above crop requirements, a leaching fraction can be calculated for sprinkler irrigated fields as:

%Leaching Requirement

In this equation, ECmax = the maximum soil EC wanted in the root zone.

This leaching fraction should be applied to coincide with periods of low soil N and residual pesticide. Again, fall is often an optimal time to move salts be low the rootzone to facilitate spring planting.

Grant Cardon
Associate Professor