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n~<~-3 tU1J:. frost-free period for Mexican Hat is over 190 days. The mean high temperature is 'over 820F. and the mean low is just under the frost threshold at 31.90F. January is the coldest month of the year in the basin, and July is the hottest. Artic fronts from Canada seldom have sufficient depth to breakover the Continental Divide. This spares the basin from the frigid blasts which occur in the high plains and front range areas east of the Divide. Air masses moving north from the Gulf of Mexico are generally diverted to the east by the high mountains of the Continental Divide and do not have a significant influence on the climate of the basin. Occasionally in the summer, the westerly winds over the Pacific Ocean will shift to the southwest and advance to the basin from the south across the Gulf of California, over the deserts of Mexico, Arizona, and New Mexico bringing hot dry air and a pain shadow effect to the San Juan Basin. The greatest precipitation in the agricultural areas of the basin occurs during the months of July through October. During these four months the basin receives nearly half of its average annual precipitation. The balance is distributed fairly uniformly by months except for June which is the driest month of the year. The snowpack which accumulates in the high mountain areas during the winter months has reached measured depth of nearly 12 feet with water equivalent of. almost 5 feet. Melting of these snowpacks causes high spring flood flows and results in sustained summer streamflows. Tributaries entering the San Juan River from the south in Arizona and New Mexico produce intermittent streamflow, mostly from high intensity, short duration, convective-type summer showers. Precipitation from widespread low intensity storms is rapidly absorbed in the desert areas and produces little runoff. The desert soils are highly susceptible to erosion because of sparse vegetative cover and low organic content. Sediment yields from these areas are high, and when runoff does occur, the desert tributaries contribute heavily to the downstream sediment load. The wide range of climatic conditions in the basin has resulted in a diversified agriculture ranging from alfalfa, grass hay, and pasture at locations of short growing seasons and cooler tempera- tures, to corn, small grain, dry beans, truck gardens, orchards. and melons in the areas of milder climates. The tourist and recreation industry have year-around benefits from the climate variations. The cool and refreshing atmosphere of the mountain parks, lakes, and streams offer much sought relief from sweltering desert and city life in the summer. High snowpacked 111-3