Friday, 14 July 2006 - 4:20 PM

Impact of Irrigation Water Quality on Infiltration in a Combined Irrigation-Rain System.

Donald L. Suarez, USDA-ARS, U.S. Salinity Laboratory, 450 W. Big Springs Road, Riverside, CA 92507, Scott M. Lesch, Univ of California, Riverside, Dept of Environmental Sciences, Riverside, CA 92507, and James Wood, USDA-ARS U. S. Salinity Laboratory, 450 West Big Springs Rd., Riverside, CA 92507.

Irrigation water suitability related to infiltration has been established primarily from short term column experiments of saturated hydraulic conductivity with waters of decreasing electrical conductivity and constant SAR. There are also a limited number of studies examining the effect of existing soil chemical conditions on infiltration of rain, but these are primarily single rain events. Conditions of combined rain and irrigation are distinctly different from that of most earlier studies and standards. The objective of the study is to establish the impact of irrigation with waters of varying SAR on water infiltration into a loam and a clay soil in a combined irrigation rain system over a one year cycle. We tested two Montana soils, Kobase Silty Clay from the Tongue River area and Glendive Sandy Loam from the Powder River area, both irrigated with 10 simulated river waters with two EC and five SAR levels and subjected to alternating rainfall for over one year. Soils were tested under bare soil conditions in outdoor containers. Soil cores were taken from the containers for saturated hydraulic conductivity tests after each season. Columns were also prepared with disturbed soil and irrigated with the 10 simulated river waters of varying SAR. Plastic containers (29 cm height and 25 cm diameter) were fitted with ceramic extractors buried in the bottom of the containers into 7 cm of fine quartz sand. A vacuum of 50 kPa (0.5 bars) was applied to the extractors before, during and after each water application but was shut off when flow ceased. Four empty containers were also positioned in 4 rows all in an open outdoor area under the rainfall simulator. The plots were subjected to alternating simulated rain and irrigation events. The simulated rain water consisted of partially deionized Riverside tap water with an EC of 0.016 dS/m. An overhead traveling rainfall simulator was designed to sprinkle rain water uniformly over the buckets, with simulated rain drop sizes of 1.6 mm in diameter with terminal velocity representative of rain. The system, delivered 0.25 cm of rain per pass. A complete rain event consisted of 2.00 L (5 cm). Passes were made in sequence to form temporary ponded conditions in order to measure infiltration times for the applied water. The simulated irrigation waters consisted of two different salinities (EC= 1.0 and 2 .0 dS/m) at SAR 2, 4, 6, 8, and 10, and one control (Riverside tap water at EC= 0.5 dS/m, SAR <1) The irrigation waters were applied on the surface (flood) at single applications of 2.00 L (5 cm). There was little difference in the infiltration results from the 2 salinities but significant differences in the SAR treatments for both soils. For the final rain infiltration event, there was no significant difference between the control and SAR 2 treatments but decreasing infiltration with increasing SAR for all SAR values above SAR 2. The decrease in infiltration ranged from a factor of 3.5 for the clay soil and 5 for the loam soil. Saturated hydraulic conductivity of undisturbed soil cores taken at the end of the experiment was measured in the laboratory using the same water compositions as used in the field. We also conducted a study with packed laboratory columns under saturated conditions. The results of these short- term laboratory hydraulic conductivity experiments are generally consistent with the results from the long-term field infiltration studies and the hydraulic conductivity measurements taken from the undisturbed soil cores run in the laboratory. In both instances the hydraulic conductivity with rain water was much lower than with irrigation water. We observed decreasing hydraulic conductivity with increasing SAR starting at SAR 2 for both the clay and loam soils. In these experiments the EC 2 treatments had a greater hydraulic conductivity than the EC 1 treatments, but only at higher SAR. The changes in hydraulic conductivity with increasing SAR were greater in the soil columns than the changes in infiltration in the field study. The procedure used in this packed column experiment is comparable to the procedures used in the earlier laboratory experiments. These results indicate that the changes in hydraulic conductivity as related to irrigation water composition from laboratory column experiments can adequately represent infiltration changes in the field. Both studies indicate that there were adverse effects on soil physical properties starting in the range of SAR 2-4.

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