A study of the effects of silty irrigation water from an intermittent stream on crops and soils in controlled plots

Corn, beans, and oats have been grown under two methods of flood irrigation. The following are the results after 3 years of treatment: 1. Corn yields from plots watered with silty water have not been significantly different from those of plots similarly watered with clear water. Corn yields from plots receiving only precipitation have been lower by highly significant differences. 2. The yields of oats have been significantly affected by sediment only when the deposit occurred before germination of the grain in quantities sufficient to stifle emergence of the seedlings. 3. Heavy silting killed out part or all of the bean plants. Placing the bean rows on ridges and selecting flows for low sediment content avoided this difficulty. 4. Yields from plots watered with flows selected for timeliness and low sediment content have not differed from those of plots watered at the same time with clear water. The sediment from these selected flows has raised the surface of the plots 3 to 4 inches in 3 years, whereas the surface of the plots receiving all flows has been raised 10 to 11 inches during the same period. 5. Application of 18 inches of water in 1939 and of 24 inches in 1940 in the "selected-flow" plots as compared to 36 inches in 1939 and 78 inches in 1940 in the "all-flow" plots produced no significant difference among the corn yields. 6. Marked increases in the clay fraction have occurred in the soils of all plots receiving silty water. 7. No great changes have occurred in the bacteria; and chemical characteristics of the soil. Although the experiment has not been continued long enough to permit the drawing of conclusions or the making of recommendations, certain points are indicated tentatively by the results of the first 3 years. From the markedly higher yields of corn and oats in the irrigated plots and the comparatively small accumulation of sediment in those plots receiving selected flows, the writers are led to the tentative conclusion that good crops may be obtained by irrigating from the relatively clearer of the intermittent arroyo flows and at the same time the land surface will not be raised to the extent of making continued cultivation of the land impracticable. On the other hand, while yields of corn and oats from the plots watered with every flow have thus far not differed significantly from those of the plots receiving selected flows, the amount of sediment deposited on these plots has been so great as to indicate that continued use of the practice could probably be impracticable except in situations where the raising of the soil surface is not a factor to be considered. This brings up the question of desilting. While this has not been tried in the present experiment, our observations of the rapidity with which basins behind diversion dams are filled lead us to believe that the cost of maintaining desilting basins would be prohibitive.