GENERAL44107 - Laserfiche WebLink

Home Browse Search

Rule 2.06.8(5)(b) of the Colorado Regulations specifically refers to potential increases in electro-conductivity of water supplying AVFs to levels above threshold value at which crop yields decrease as constituting material damage. The rule further cites a specific paper by E.V. Maas and G.J. Hoffman of the U.S. Salinity Laboratory published in the Journal of the Irrigation and Drainage Diversion, ASCE., June, 1977. In their paper, Maas and Hoffman list research derived salinity threshold levels and response rates for selected agricultural species and group those species as well as a number of additional crops into four categories based on relative tolerance to salinity (sensitive, moderately sensitive, moderately tolerant and tolerant). The Division has taken the approach that a positive material damage finding would be made if projected salinity increases resulting from mining would result in significant decreases in production or shifts in species composition based on the Maas and Hoffman paper. On the Trout Creek AVF, the applicant sampled vegetative cover within a number of farmed fields as denoted on Supplemental Map No. 1. Vegetative production, as well as cover was sampled in one field, Pasture 11. A majority of the species on the AVF would be considered moderately tolerant, and on an acreage weighted basis only 18.4% of the cover was made up of moderately sensitive species (salinity threshold reached at root zone soil saturated paste extract electro-conductivity of 1.5 mmhos/cm). A number of the species encountered have not been ranked by Maas and Hoffman, and other references were consulted. In general, Aveneae tribe grasses and Trifolium species were considered to be moderately sensitive, while r-Titiceae, Stipeae, and Poeae tribe grasses (with the exception of Dactylis) were considered to be moderately tolerant. Carex and Juncus species were also considered to be moderately tolerant. As shown in TABLE 17 of this document, projected Trout Creek water quality for flood irrigation is approximately .5 mmhos/cm. Assuming that a root zone saturated extract soil salinity of .75 mmhos/cm would result from flood irrigation water with a specific conductance of .5 mmhos/cm, no decrease in crop production would occur. The assumed relationship between irrigation water salinity and root zone soil salinity is within the typical range reported in USDA Handbook 60. Based on leaching calculations included in the application and soil sampling conducted on the Fish Creek AVF by the Division, the 1.5:1 relationship is considered conservative. No decrease in production of any of the species currently grown on the Trout Creek AVF would be expected unless the conductivity of the irrigation water were to exceed 1 mmhos/cm. Based on species composition data submitted, significant production decreases (13% for a farm unit) would not occur unless irrigation water salinity were to exceed 1.5 mmhos/cm. In summary, projected flood irrigation water quality is .5 mmhos/cm and material damage would not occur unless water quality were to exceed 1.5 mmhos/cm. -21-