SALINITY AND TRACE ELEMENTS ASSOCIATED WITH WATER REUSE IN IRRIGATED SYSTEMS: PROCESSES, SAMPLING PROTOCOLS, AND SITE-SPECIFIC MANAGEMENT
Location: Water Reuse and Remediation
Title: Salinity control on irrigated land and use of saline water for irrigation
Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: September 1, 2008
Publication Date: September 7, 2008
Citation: Suarez, D.L. 2008. Salinity control on irrigated land and use of saline water for irrigation. Meeting Abstract. Given at workshop "State of the art of salinity control and use of low quality waters in irrigated areas" held in Santiago del Estero, Argentina September 9-11, 2008 p. 1-2.
Current irrigation practices in arid and semi-arid regions throughout the world are not sustainable. These regions are experiencing increasing population and development with increasing demands for limited fresh water for municipal and industrial use. In arid areas fresh water use is currently already in excess of sustainable quantities. Irrigated acreage in the western U.S. and many other regions is already declining due to reductions in delivered water. Soil salinization is still increasing throughout the world. Improvements in irrigation efficiency and leaching control are possible and essential, but they provide only a partial solution to sustaining irrigation and its high crop production.
Almost all arid regions have abundant quantities of low-quality saline, drainage and sewage waters, most of which could potentially be used for irrigation. Use of these waters for irrigation is possible but requires new strategies for water management including new knowledge of factors affecting infiltration and crop production. We also need the development of models that consider the numerous interactions of chemical, physical and biological processes, enabling evaluation of various management practices. We need to develop alternative crops, and new varieties that are tolerant to salinity, ion imbalances and toxic elements.
Water quality criteria were developed with the idea of being simple and designed to avoid problems in most instances. We currently reject waters that in some instances can be detrimental and without consideration that in many areas the water requirements can be met by a combination of rain, fresh water and saline water, thereby diminishing the salinity impact. In many instances use of recycled and brackish water, currently considered unsuitable, will result in some reduction in potential yield. Nonetheless this can be acceptable and desirable if yield is considered in the context of society needs and grower profitability. Treated wastewaters have elevated pH, alkalinity, and sodium, relatively low Ca/Mg ratios, high concentrations of dissolved organic matter, all adverse to infiltration and soil structure, as well as ion imbalances and potentially toxic elements. Use of these waters may require periodic application of amendments and/or leaching, utilizing new knowledge about factors affecting infiltration and crop production. Environmental concerns about recycled water include plant uptake of toxic elements, pharmaceuticals, endocrine disrupters etc. as well as off-site impacts to discharge areas.
Existing guidelines consider only salinity and sodicity (SAR). The high variability in soil stability is related to the many factors that affect stability, among them pH, oxide content, organic matter, tillage, residue management, soil texture and clay mineralogy. Other factors that affect water suitability include irrigation system, rainfall-quantity, intensity and seasonal distribution, evoptranspiration demands, soil hydraulic conductivity, drainage system, uniformity of irrigation and acceptable relative yield- an economic consideration.
Water suitability for irrigation and sodicity hazard related to infiltration has been established primarily from laboratory experiments, almost all based on short term column experiments of saturated hydraulic conductivity with waters of decreasing electrical conductivity (EC) and constant sodium adsorption ratio (SAR). Fortunately these data correlate with longer term infiltration experiments. We recently conducted year- long outdoor studies with conditions of combined simulated rain and irrigation and wetting and drying cycles with waters of varying SAR and electrical conductivity of either 1.0 and 2.0 dS/m, and varying pH. Rain has an adverse impact related to the SAR of the soil water at the time of infiltration. Based on these studies, we conclude that when considering rain as well as irrigation water, there is no threshold SAR value at which there is a reduction in soil infiltration. Any increase in SAR above the control results in a reduction in infiltration. Similar results, but with less infiltration rate loss, have been observed in experiments with irrigation only. This does not mean that waters above SAR 2 are not useable; it means that we must consider the site specific reaction to the less than ideal water. For example, for sandy soils a 20% reduction in infiltration over the course of a year is not significant but for a clay soil with limited infiltration, it could result in a reduction in water availability and crop yield. These evaluations can be assisted by the use of modeling simulations.
Recent studies have also determined an adverse effect of increased irrigation water pH on soil infiltration, with the effects increasing with time. Degraded waters often contain elevated concentrations of minor elements such as boron that may adversely affect crop growth. In many instances use of these waters may be judged unsuitable based on current steady state considerations however transient model simulations suggest conditions under which they may be used. Examples are given for Unsatchem model simulations using high boron waters for irrigation along with suggestions for optimal management.
Computer models are also very useful for optimizing saline and sodic soil reclamation and minimizing amendment applications and time. These recommendations can be site specific if we combine the modeling with detailed salinity maps obtained by remote sensing (EM) technology.
The salinity status (EC) of soils as well as soil SAR is most commonly reported in terms of the saturation extract. This is considered by most to provide a good reference water content for comparisons among experiments and field conditions. The salt tolerance of crops is also reported in terms of the saturation extract EC. Alternatively other standards such as 1:2 and 1:5 soil:water extracts are also utilized in some regions, and water standards have been developed in terms of those extracts. Plants respond to soil water EC and not the measured EC of a diluted, reference water content. Simple conversions of soil water (generally at field capacity) to saturation extract exist but can lead sometimes lead to significant errors. These errors are even larger for extracts at higher water contents and especially for gypsum containing soils. This salinity over-estimation leads to over-estimation of salt damage in gypsum containing soils. It is likely that the plant responds to the time integrated salinity- this consideration means that for infrequent irrigation cycles soil salinity is greater than estimated. As demonstrated, these factors can be evaluated using the Extract Chem. model that converts water composition as a function of water content.
Extent and timing of rain needs to be considered when evaluating suitability of waters for irrigation. At the present time, rain interactions are generally but incorrectly ignored. As a first approximation we can consider that crops respond to the average of the rain and irrigation water composition, but salt sensitivity as related to stage of growth must be considered. Because salt tolerant crops are generally lower value crops, and often lower yielding crops, they should not be automatically recommended for saline conditions. Despite some yield loss moderately salt tolerant crops such as alfalfa may out-produce more salt tolerant crops such as wheatgrass at salinities up to 15 dS/m.