Hyrdology of the Valley
The San Luis Valley qualifies as a high mountain desert and is one of the most productive agriculture areas in Colorado. Precipitation on the Valley floor is approximately seven inches per year. Precipitation on the Mojave Desert is only slightly less at five and one-half inches per year. How can this be?
The geological formation of the Valley has provided high mountain ranges around its edges that receive significant snow in the winter which then melts and flows together with water from summer rains into the Valley through streams and rivers. These mountains form a watershed of approximately 4,700 square miles. Water from these streams is then diverted by ditches and canals which provide irrigation water to crops on the floor of the Valley. Most of the streamflow is derived from snowmelt and averages about 1,500,000 acre-feet per year. One acre-foot of water would cover one acre of land one foot deep.
The above explanation would seem to answer the question posed in the first paragraph, but there is more to the story. The rest of the story surrounds the use of wells in the Valley.
First, it is useful to obtain a basic understanding of why high volume wells exist in the Valley and where the water is derived that wells draw from the underlying groundwater. The San Luis Valley is located within a geologic feature called the Rio Grande rift. This rift can be visualized as a trough probably resulting from the earth’s crust pulling apart resulting in stress faulting and down dropping of a block of the crust. This several 1,000 foot deep trough extends in a nearly north-south direction generally along the center of the Valley.
Through the erosional process over millions of years in the nearby mountains, this trough has been largely filled with sand, gravel and clay layers. It is likely that many of the clay layers were formed through a soil evolutionary process with a large part of the process occurring at the bottom of a lake that covered the Valley floor. In 1822 trapper Jacob Fowler wrote in his journal of the probability of a lake, similarly in 1910 C.E. Siebenthal studied the Valley and described evidence of a historic lake, and finally U.S. Geological Survey investigators in 2007 published a report concerning ancient Lake Alamosa. The combination of erosional material filling this rift trough and Lake Alamosa’s existence created a very large aquifer system into which wells were drilled beginning in the 1880’s.
The early wells tapped into the artesian aquifers and the wells free flowed without pumping. The artesian aquifers were located one-hundred or more feet below the surface beneath clay layers. The clay layers were formed in the rift trough and lake bottom with the edges near the mountain being higher than under the Valley floor. Water seeps from streams and rivers as they flowed into the Valley with a portion of the water flowing below these clay layers. The largest sources of inflow to the Valley are the Rio Grande and Conejos River. This configuration of the clay layers with water flowing below these layers at higher elevations near the outside edge of the Valley caused the pressure and resulting free flow from wells in lower portions of the Valley.
The sand and gravel that filled this rift trough also created a shallow groundwater aquifer that extends from near the ground surface to the top of the clay layers below which the artesian aquifer exists. Commonly the shallow aquifer is called the unconfined and the deeper artesian aquifer the confined. Availability of water to supply large wells tapping the unconfined aquifer is variable throughout the Valley. Many of the large capacity unconfined wells are near the Rio Grande and northerly of the Rio Grande a few tens of miles.
After approximately the 1940’s and extending into the 1970’s and somewhat later in some areas of the Valley, extensive well development occurred as an important source of irrigation water. By the early 1970’s, the Colorado Water Resources Circular No. 18 estimated that more than 1,800 large-capacity irrigation wells withdrew water from the unconfined aquifer. Water was being withdrawn from the confined aquifer by more than 650 large-capacity irrigation wells and about 7,000 small-capacity domestic, stock and pasture irrigation wells.
Since the beginning of irrigated agriculture in the Valley, flow patterns of the streams and rivers experience peak flows during the spring snowmelt. This natural pattern has made it difficult to irrigate crops that have maximum water needs during the middle to late summer. Construction in the mountains of reservoirs in the early 1900’s was very valuable in extending the availability of surface water, but their limited size has not permitted full capture of high spring flows. An early effort to provide a solution to summer shortage of surface water was the practice of sub-irrigation where the groundwater levels were raised in the root zone of crops early in the summer when surface water was more available and then this “store” water could be used by crops later in the season.
Upon the development of large capacity wells, it has been recognized that using wells to withdraw water from the large groundwater reservoir under the Valley floor could solve this middle to late summer shortage of surface water and permit maximum crop production. Through seepage from ditches and canals and recharging the shallow groundwater aquifer in the area northerly of the Rio Grande with seepage from ponds filled with water from ditches, this groundwater reservoir could be operated much like a surface water reservoir. The reservoir is filled through seepage during the high river and stream flow periods in the spring and then withdrawn by wells later in the summer.
The use of both surface water and groundwater to supply irrigation for crops would seem like an ideal solution. However, groundwater level measurements indicate the storage volume in the underground aquifer system has declined to unsustainable levels and groundwater hydraulic studies indicate that operation of wells is causing depletion of the streams and rivers that supply surface water rights that are senior to well rights. In summary, well usage has grown too large and reductions are required. Further, well depletions to surface water rights must be replaced. This is the primary reason the State Engineer’s Office, the agency that administers the both surface and wells water rights, is in the process for developing rules and regulation for wells located within the San Luis Valley.