Water, Hydrology, and Climate

The science of hydrology deals with properties, distribution, and circulation of water on the surface of the land, in the soil and underlying rocks, and in the atmosphere. Certainly, water is one of the most important natural resources in the San Luis Valley. It is vital to every living thing; nothing can survive without it.

To better understand how water relates to the dunes, look at the elevation and climate in the area. Elevations vary from around 7,500 feet in the center of the San Luis Valley to over 14,000 feet in the Sangre de Cristo Mountains. Summers are short and cool, while winters are long and cold. The annual precipitation is over 40 inches in the higher elevations, while the valley floor gets much less. Alamosa receives an annual average of six inches and the Great Sand Dunes annual average is ten inches.

The reason why the valley floor receives so little rain, while the higher elevations receive so much more, is due to the "rain shadow effect." The entire valley is under a rain shadow. As eastward moving clouds rise over the San Juan Mountains, they cool. Cool clouds cannot carry as much moisture as warmer air can, so the moisture is dropped on the San Juan Mountains in the form of rain and snow. Notice in the map at right, that over 50 inches of precipitation can fall in the San Juans per year. As the clouds continue east, descending and moving across the valley, they warm and hold moisture until they reach the Sangre de Cristo Mountains. They cool again as they climb over the mountains and once again release precipitation.

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