OCR Text |
Show -11- San Luis Valley the subsurface (underground) method is practiced. The following variations obtain: Mean Area Altitude Growing Season Ann. Temp. Poudre Valley 5,000 135 days 48° San Luis Valley 7,700 110 " 42° The different methods of applying water stand out in bold contrast. In the Poudre Valley the alfalfa and grain crops (occupying the greater percent of the irrigated acreage) are irrigated by flooding the surface of the fields during the hot summer months, and the "row crops" (potatoes,, beets, onions, cabbage, etc.) are irrigated by running the water between the rows in surface furrows. Thus the water is subjected to the maximum air and heat exposure, and the maximum capillarity results from the exposed flooded surfaces. Conditions conducive to the highest evaporation loss obtain. There is no subsurface irrigation. Even the "corrugation method," by which the water is applied to the alfalfa and grain crops in small parallel furrows, is never practiced. The surface mulch necessary to destroy capillarity, and so essential to preservation of soil moisture, is destroyed and never returned (except to a limited extent with row crops) during the entire season. The soil blanket is the secret of successful "dry farming." Without it the soil moisture soon exhausts into the air. With it crops are grown successfully. And yet, notwithstanding the fact that the evaporation losses are compensated with additional applications of water in the Cache la Poudre Valley, the ascertained consumptive use is low, and we know that more complete studies will disclose still lower values than those already ascertained. In the San Luis Valley a large part of the irrigation is by the subsurface method. The water plain is annually |