Data Requirements and Preliminary Results of an Analog‐Model Evaluation—Arkansas River Valley in Eastern Colorado^a

Abstract

The intensively irrigated Arkansas River Valley in Colorado is underlain by a valley‐fill aquifer resting in a U‐shaped trough cut in relatively impermeable Cretaceous rocks. Ground water is pumped to supplement surface water; in the last 10 years pumping has more than doubled. Ground water is closely related to the Arkansas River; percolation from irrigation recharges the aquifer, which discharges into the river. Pumping has resulted in a reduction in streamflow because it intercepts water that ordinarily would have reached the river. The 1,500 irrigation wells in the Arkansas Valley withdrew 230,000 acre‐feet of water in 1964. An analog model is being used to evaluate the relation of ground water to surface water and to predict effects of changes in water management. The model, simulating a 150‐mile reach of the Arkansas Valley (Pueblo to the State line), has a resistor spacing of 8 per modeled mile. The framework for the model was a transmissibility map; transmissibility ranges from less than 50,000 to 700,000 gallons per day per foot. Specific yield averages about 0.2. Hydrologic boundaries, such as the Arkansas River, and the bedrock valley‐fill contact were simulated. Applied water, precipitation, evapotranspiration, and ground‐water pumping were the independent variables programed. The model is being verified by comparing predicted changes in water level and river discharge with observed changes.

Assessment of Long‐Term Salinity Changes in an Irrigated Stream‐Aquifer System

“…The Fort Lyon Canal is unlined and parallels the Arkansas River along the northern edge of the valley throughout the study area. The bedrock beneath and adjacent to the alluvium consists mainly of low-permeability shales with some interbedded limestones; more detailed descriptions of the local and regional hydrogeology are presented by Hurt and Moore [1972] and Moore and Wood [ 1967].…”

Stream Depletion in Alluvial Valleys Using the SDF Semianalytical Model

“…Documentation of the numerical modeling used in generating the USGS SDF maps (e.g., Hurr and Schneider 1972a, 1972b) is limited, but based on descriptions in Moore and Wood (1967), Jenkins (1968a, 1968b), Moulder and Jenkins (1969), Hurr and Schneider (1972b), Jenkins and Taylor (1972, 1974), Missouri Basin States Association (1982), Warner et al (1994), and the recollection of USGS personnel (R.R. Luckey, personal communication, 2005), the maps account for spatially variable transmissivity, the presence of stream and aquifer boundaries and their irregular shapes (e.g., meandering streams and the presence of tributaries), and the location of the well with respect to the stream and aquifer boundaries.…”

On Modeling Philosophies

“…It was frequently difficult to simulate virgin recharge and virgin discharge in the analog model, and in practice, it was almost never done (Moore and Wood 1967). The model replaced the analytic method; the intent of the model was to determine capture directly.…”