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{ CATI , CIT , CIT - Research Publications }
- Research Notes -
Integrated On-farm Drainage Management:
Using Plant Transpiration to Reduce Drainage Volumesby Sharon Benes, Doug Peters and Steve Grattan
CATI Publication #990602 © Copyright June 1999, all rights reserved
INTRODUCTION
High water table and soil salinity are chronic problems for many West-side San Joaquin Valley growers. Subsurface, or "tile" drainage as it is often called, provides water table management and the ability to apply excess water for the leaching of salts from the soil. However, utilization of subsurface drainage has been limited by environmental restrictions on the discharge and storage of drain water. These regulations arose from high concentrations of selenium which sometimes occur in drainage and are poisonous to waterfowl if bioaccumulated in the food chain.
Consequently, a means of reducing drainage volumes is desirable. Reutilization of the drain water to irrigate salt tolerant crops and other non-crop species has been proposed because plant water use (ET) reduces drainage volumes and concentrates salts in the water, thereby facilitating final evaporation in a solar evaporator. This is a more environmentally acceptable method than storage of drain-age in evaporation ponds which often attract waterfowl.
Broadly defined, "drainage water re-use" is the utilization of subsurface drain water to irrigate crops. In practice, this includes the following options; however this article will focus only on sequential re-use (#4).
1) Application of drain water to an undrained field cropped to a salt tolerant row crop or forage "Jose" tall wheat grass and other forages have been used for this purpose, but ideally, a blend of a warm and cool season forage would be best. This practice is not sustainable in the long-term, as soil salinity in the undrained field tends to increase beyond the tolerance limit of the forages.
2) Blending Saline drain water is mixed with good quality (canal) water to achieve irrigation water of suitable quality for the salinity tolerance of the crops being grown. The blending must not unduly compromise the quality of the irrigation water. Broadview Water District in western Fresno County has utilized blending.
3) Cyclic Re-use Saline drain water is used for irrigation but alternated with high quality water, often within a multi-year rotation. The drain water is used only for the more salt tolerant crops of the rotation and/or during the more salt tolerant stages of crop development (e.g., after first bloom for tomato or after thinning for cotton).
4) Sequential Re-use Drain water is reused several times, each time being applied to progressively more salt tolerant plants. The objective is to reduce the volume of the drainage before its discharge into a small acreage solar evaporator. A very small portion of the farm, usually the least productive ground or an area with high water table problems, is designated as the "Re-use Area." With sequential re-use all drainage and salt is managed on-farm. Drainage lines must be installed in the reuse area, so that salts can be moved through the system and finally, into the solar evaporator.
From this point on, we speak of drainage water re-use only in the context of the context of the sequential re-use system.
Many possibilities exist for combining crops and non-agronomic plants in a sequential, drainage water re-use system. At each step in the re-use, species selection would be determined by plant tolerance to salinity and trace elements such as boron, and secondarily, by economic potential and farmer preference. For example, the first (primary) drainage might be applied to a salt tolerant crop such as cotton, sugarbeets, or canola; the secondary drainage to a highly salt tolerant forage such as bermuda-grass; the tertiary drainage to halophytes (plants native to highly saline environments); and lastly, drainage from the halophytes would be discharged into a solar evaporator.
In sequential re-use, the volume of the drainage is reduced by crop water use (evapotranspiration or ET). After each re-use the water is further reduced in volume, but more concentrated in salinity, because plants transpire only freshwater and salts stay behind in the soil water. By reducing the drainage volumes prior to discharge, a solar evaporator can be utilized for the storage of the last drainage rather than an evaporation pond. The acreage requirement for a solar evaporator is about one percent, or less, of the irrigated acreage, as compared to 10 percent when a conventional evaporation pond with no drainage re-use is operated.
RESEARCH ON HALOPHYTE "ET"
Our efforts have focused on the final stage in the drainage water reuse system, when the concentrated, highly saline drainage is applied to halophytes. Halophytes are plants that are native to saline environments such as coastal salt marshes, and they thrive on salinity. These plants are the only choices for irrigation with concentrated drain water. All of our "crop plants" are non-halophytes and cannot survive at the salinity of concentrated drainage, which can reach two-thirds the salinity of seawater. However, at the low end of the salinity range of concentrated drainage, the more salt tolerant forages such as bermudagrass, siltgrass and Puccinellia, which are not halophytes, may survive and grow reasonably well.
It is important to emphasize that the cultivation of halophytes is primarily to achieve the final reduction in drainage volumes: economic return would be a bonus, but not a requisite. Profits are likely to increase, however, in the main portion of the farm where the utilization of subsurface drainage can reduce water-logging and salinity problems, thereby enhancing yields and possibly extending crop choices to higher value, salt-sensitive species.
More research is needed to demonstrate that the halophytes proposed for re-use cropping systems will thrive under irrigation with the hypersaline, high boron, drainage water of the San Joaquin Valley, and that under these conditions, their water use rates will be sufficiently high to provide the needed reduction in drainage volumes. Our research effort has therefore focused on water use (ET) measurements for the halophytes.
Most halophytes are undomesticated plants, having only modest agronomic potential at present. Amongst those with greater (or more immediate) potential is pickleweed (Salicornia bigelovii). Salicornia has potential as a seed oil crop (an oil similar to soybean), as biomass for particle board construction, and as animal forage or feed if it is mixed with materials of lower salt content. Saltgrass (Distichlis spicata) and quailbush (Atriplex nummularia) are other halophytes having forage potential. Several other forages, such as bermudagrass, siltgrass (Paspalum spp.) and Puccinellia, also show promise, but at lower salinities than those tolerated by true halophytes. These salt tolerant forages have better economic potential than the halophytes, in part because of lower salt content in their tissue.
Results to date
Our research over the past three years suggests that the inclusion of halophytes in drainage water re-use systems has the potential to significantly reduce drainage volumes. We measured high water use rates for two of the candidate halophytes, Salicornia and saltgrass, under irrigation with concentrated drainage effluent from fields near Mendota, California.
The drain water applied to these halophytes was the second collection of drain water. Salinity was 30 dS/m (about two-thirds seawater strength) and boron was greater than 25 ppm. In that area, canal water had been applied to row crops; the primary drainage was collected and applied to Eucalyptus trees, and the drainage from the trees (secondary drainage) was applied to the Salicornia and saltgrass.
We measured the halophyte ET using sunken containers in the field that function as drainage lysimeters. Plants are established in the lysimeter, which contains sand to facilitate drainage. Heavy West-side clays cannot be used in the lysimeters because of their poor drainage. The lysimeters are irrigated several times per day and all drainage is pumped back to the source tank. A nightly refill replaces the water lost during the day due to ET. The refill is metered and provides the measurement of water use. The system must be leak-proof to allow ET calculation by this method.
The ET rates that we measured for Salicornia irrigated with the highly saline drain water from Mendota in 1997 met or exceeded CIMIS Eto (reference water use for a well-watered grass, nonsaline conditions) for much of a four month summer period (Fig. 1). Although encouraging, this was not a direct comparison of Salicornia and nonsaline grass ET because the Salicornia estimate came from our lysimeter system (volume balance calculation) and the CIMIS ETo is a micrometeorological (weather station) estimate.
Consequently, in 1998 we planted fescue into some of our lysimeters. The fescue provided an "on-site" estimate of nonsaline grass ET the complement of CIMIS ETo but measured using the same lysimeter system used for halophyte ET. In 1998, the ET of saltgrass irrigated with concentrated drain water from Mendota was also measured. For much of the season, saltgrass ET averaged about 60 percent of the ET of the fescue receiving only canal water for irrigation (Fig. 2). For example, from Aug. 22-29, nonsaline fescue ET was 11.4 to 12.9 mm/day and saltgrass irrigated with saline drainage had ET rates of 7.1 to 8.6 mm/day. Because Fig. 2 shows cumulative ET, the difference between saltgrass ET and fescue ET increases as the season progresses (lines diverge).
In 1998, we discovered that our estimate of fescue ET was considerably higher than the CIMIS estimate of nonsaline grass ET (ETo, data not shown). A possible explanation is that incomplete vegetative cover in the area surrounding our lysimeters increased advective forces and resulted in higher water use. This was not a problem for the comparison of saltgrass ET to fescue ET because the same sparse cover surrounded the saltgrass lysimeters. We are striving to reach full cover around the lysimeters at our new site and will continue to compare the relationship between our lysimeter-based ET measurement and the weather station method utilized by CIMIS. Fescue will be included in future lysimeter plantings to provide the on-site estimate of nonsaline grass ET, against which we will compare the halophyte water use.
From the ET data obtained for saltgrass in 1998 and for Salicornia in 1997, we conclude that both halophytes grow well under irrigation with concentrated drain water, and both maintain water use rates of at least half of those of a nonsaline grass. Irrigation with the San Joaquin Valley drainage water results in saline conditions quite different than the native seawater environment of the halophytes a larger fraction of sodium sulfate, and much higher levels of boron (Table 1). Notwithstanding these differences, both Salicornia and saltgrass thrived and transpired relatively large amounts of water.
1999 research
Re-location of our field site from Mendota to Red Rock Ranch near Five Points, California, and the expansion of our lysimeter facility will allow us to measure ET over a larger vegetated area and to include some of the salt tolerant forages also under consideration for use in drainage water re-use systems. This year we will measure ET for bermudagrass irrigated with moderately saline drain water (10-15 dS/m) and for the halophytes Salicornia, saltgrass, and Atriplex irrigated with concentrated drain water (28-30 dS/m). Fescue under nonsaline conditions will also be included.
Oster & Kaffka (1999) proposed that drain water be considered as a water resource for forage production. Experiments in sand tanks at the USDA Salinity Lab in Riverside and in field plots at Westlake Farms (Strattford, California) show good biomass production and acceptable forage quality (ADF and NDF) for common bermudagrass, siltgrass (Paspalum vaginatum) and the saltgrass under irrigation with drain water up to 20 dS/m. For bermuda and siltgrass, biomass production began to decline slowly as the salinity of the drainage used for irrigation exceeded 20 dS/m.
Forages are considered ideal for the middle phase of the sequential DW re-use system. They are attractive because (1) forages have greater economic potential than do halophytes; (2) forage production in the West-side San Joaquin Valley is said to be insufficient to match the increase in dairies, especially in Tulare county; and (3) natural resource agency personnel would like to move livestock grazing out of the Sierra foothills to minimize grazing impacts on native vegetation.
REFERENCES
1. Glenn E.P., Brown J.J. and OLeary J.W. (1998). Irrigating Crops with Seawater. Sci. Am. Aug. 98. Pp. 76-81.
2. Oster J.D. and Kaffka S.R. (1999). Forage production using saline-sodic drainage water. Proceedings of California Plant and Soil Conference, CA chapter of the American Society of Agronomy (ASA) and California Fertilizer Association (CFA). Jan. 20-21, 1999. Visalia, CA. pp. 33-37.
3. Oster J.D. (1997). Impact of alternative crops on salt disposal strategies. Proceedings of California Plant and Soil Conference, CA chapter of the American Society of Agronomy (ASA) and California Fertilizer Association (CFA). Jan. 15-16, 1997. Visalia, CA. pp. 38-42.
4. San Joaquin Valley Drainage Program (1990). A management plan for agricultural subsurface drainage and related problems on the Westside San Joaquin Valley. 183 pp.
5. San Joaquin Valley Drainage Implementation Program (1999a). Final report of the Drainage Reuse Technical Committee. S.R. Grattan, chair. San Joaquin Valley Drainage Implementation Program and the University of California Salinity/Drainage Program. 81 pp.
Copyright © 2000. All rights reserved.
CALIFORNIA AGRICULTURAL TECHNOLOGY INSTITUTE - CATI
College of Agricultural Sciences and Technology
California State University, Fresno
