The quality of water from Montana's rivers and streams generally decreases as the irrigation season progresses. Historic records indicate that sediment load is usually heaviest during the May and June snowmelt runoff periods and as the sediment load decreases and stream volumes decrease, the salt level of most rivers and streams increases. This is particularly true to the streams and rivers east of the Continental Divide. With that being the case, a few guidelines for irrigating with salty water might prove useful for irrigators, especially those using water from the Powder, Milk, Marias, Tiber, Musselshell, Tongue Rivers and secondary tributaries to the Missouri and Yellowstone Rivers.
Irrigating with saline (salty) water. The smaller rivers that supply most of the irrigation water for the eastern two thirds of Montana are interesting because the saline and volume change dramatically during the year. Review of the historic water quality and flow records indicate several changes through the year. Knowledge of these changes may be of some value in attempting to "get the most" our of the available irrigation water.
§ fill the profile as early as possible after the peak flood stage or as soon after harvest as possible, if you are harvesting forages (hay, alfalfa). The sediment and salinity levels tend to increase beginning early in the spring as runoff begins. Salinity level peaks and then starts to decrease as dilution takes over, while sediment continues to increase. During the high flow period salinity is low, but sediment is high. Sediment generally tends to start decreasing dramatically after the peak.
§ if at all possible, delay irrigation until after the peak flow period, as the flow level begins to drop. Salinity and sediment tend to be lower at this flow rate on the "down" or falling stage than the same stage on the "up" or rising level part of the cycle.
§ some sediment in the water will help move the advancing wetting front across border‑dike, graded border, basin, and furrow irrigated fine sandy loam soils.
Salinity (or salt load) is best determined by measuring the TDS (total dissolved solids) or EC (electrical conductivity). The TDS varies from as low as only a few hundred parts per million (or milligrams per liter), to as much as 2500 to 3000 parts per million (mg/l) during the lowest flow periods in some of the smaller streams of eastern Montana. The lowest TDS usually occurs when the river level has risen to its maximum and is then falling. When the river goes through rising and falling stages due to rain (especially thunderstorms), the TDS is usually lower at a given river level when the level is falling, rather than rising.
Irrigation strategies to reduce salt load. If possible, when irrigating in the spring and early in the irrigation season, fill the profile with water of low TDS/EC. The EC of the water will generally be between 0 and 9 mmhos/cm (equal to 9000 micromhos/cm). To convert this value to an approximate TDS (total dissolved solids), multiply the EC in mmhos/cm by 640. The result will be an approximate TDS in milligrams per liter. The soil will tend to concentrate salt during the irrigation season and thus have an EC greater than the irrigation water. In well drained soils, the EC will be about the same all the way through the root zone, while on poorly drained soils, the EC will generally increase dramatically with depth. Young plants and seedlings are much more sensitive to EC of both the irrigation water and soil than established plants. Once the EC gets above about 2.4 mmhos/cm in the soil, plants will begin to show signs of stress . . .stress that looks just like drought.
Irrigation strategies to reduce salt injury to seedlings. On new plantings of alfalfa, grass, other legumes, small grains, corn, and sorghum, shorten the length of time or your sets early in the season, when plants are still small, and irrigate just a little more frequently. You'll pump just the same amount of water as before, but get a better stand, better early growth, and an increase of as much 25% in overall yield. In addition, the irrigation water will tend to have slightly less dissolved salt this time of the season.
Use of saline water for irrigation. The following is a summary of an article by J. D. Rhoades, soil scientist with the U.S. Salinity Lab in Riverside, CA. The article first appeared in the October 1984 issue of California Agriculture. Saline water, or water which is generally classified as having too much dissolved salt for irrigation, can often be used successfully without hazardous long term effects on the crops or soils. However, certain conditions need to be met:
§ the soil being irrigated must be well drained
§ salt tolerant crops (established alfalfa, barley, sorghum, sudan grass, sordan) should be the primary crops grown
§ rotations should be planned to provide for a sequence of progressively more salt tolerant crops
§ salts should be leached out of the soil in the spring or winter
§ as the salinity of either the irrigation water or soil solution increases (with prolonged crop water use and through the irrigation season), the volume of irrigation water applied should be progressively increased.
As Rhoades points out, adoption of new crop and water management strategies can further facilitate the use of saline water for irrigation. One strategy is to substitute more saline water (later in the irrigation season) for good quality water to irrigate certain crops in the rotation or well drained soils. Whatever salt build up that might occur in the sol from irrigating with salty water can be reduced in the following winter or spring from rainfall or irrigation with low salinity irrigation water.
Soils do not usually become excessively saline from use of saline water in a single irrigation season. It may even take several irrigation seasons to affect the level of salt in the soil solution. The maximum soil salinity in the root zone that results from continuous irrigation with saline water does not occur when salty water is used only a fraction of the time.
For purposes of comparison, Colorado River irrigation water has a TDS of about 900 ppm, while the rivers of central and eastern Montana generally ranges from about 750 to 1,500 ppm during the irrigation season. Drainage water TDS will usually be 3,500 to 4,500 ppm.
Categories: Irrigation Management, Water Quality
Date: 1995-1996