ArkValley Irrigation Grant Final Report

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I I I I I I I I I I I I I I . . ~ Demonstration of Irrigation Technology to Improve Crop Yields, Returns and Water Quality in the Arkansas River Valley of Colorado SUMMARY AND CONCLUSIONS The Arkansas River in southeast Colorado is one of the most saline rivers in the United States. The average salinity levels of the canal systems along the river increase from 300 ppm total dissolved solids (TDS) near Pueblo to over 4,000 ppm near the Colorado-Kansas border. As a result, salinity levels on cropland being irrigated with these waters are higher than desirable. Crop yields on several thousand acres are being reduced as a result of these high salinity levels. Two basic processes responsible for the high salinity levels are salt pickup and salt concentration. Salt pickup occurs from water flowing over saline and sedimentary materials, from erosion of saline soils, from deep percolation through saline soils and from groundwater flow through saline sedimentary deposits. Irrigated agriculture causes salt pickup by water flowing over and percolating through salty soils. Salt concentration is significantly increased by consumptive use due to human activities. "Consumptive use alone causes a seven-fold increase in the salt concentration in the Arkansas River." (Ref. #2) Evapotranspiration by crops and from weeds and phreatophytes in waste water areas are important consumptive uses as well as evaporation from reservoirs, canals, high water table areas and from cropland receiving excessive amounts of irrigation water or poorly timed irrigations. Municipal and industrial activities also contribute to the consumptive use ofthese waters. An additional problem is nitrates from highly fertilized crops are being carried to the Arkansas River as tailwater return flow as well as through the root zone by deep percolation. This is the result of over- irrigation due to inefficient methods of water application. New irrigation conservation technology as low energy precision application (LEPA) through center pivot systems with drop hoses and Low Drift Nozzles was used in the demonstration to improve irrigation effectiveness and reduce labor. Surge irrigation with the use of computerized, solar powered valves was successfully demonstrated in 1996 but was abandoned in 1997 due to continuing pump and labor problems. Sorghum-sudangrass being surge irrigated produced equal yields of 6 tons per acre but used up to 25% less water as compared to conventional irrigation. Initial soil tests indicated an extremely high salt content on both of the fields being used on this project. Analysis of composite soil samples from each of the two fields used in the demonstration showed soil electrical conductivity (EC), a measure of salts, at the surface foot in Pivot #7 averaged 5.2 mmhoslcm while EC ranged from 4.7 mmhos/cm on the Apishapa silt clay to 6.9 mmhos/cm on the Bloom silt loam in Pivot #8. Sub soil samples in Pivot #7 ranged from a low of6.1 at the 10 foot level to a high of8.8 mmhos/cm at the 3 foot level. EC on the Bloom silt loam was 13.6 mmhos/cm at the 2 foot level while staying in the 7.3 or less in the Apishapa silty loam. I