Semiquantitative Analysis of Water Appropriations and Allocations in the Upper Rio Grande Basin, Colorado

A semiquantitative analysis was used to assess the impacts of water rights appropriations and allocations in the Upper Rio Grande (URG) Basin, in Colorado. The study also explores the causes and effects of the changes on collateral elements in the URG agriculture system, namely the San Luis Valley (SLV) in Colorado. Population increases, after the acquisition of the territory from 1854 to 1900, were the major cause of increased acquisitions of surface water rights. By 1912 surface waters were nearly 100% appropriated. The population continued to increase until 1930 after which it remained stable. Water users began making large increases in the number of appropriations of groundwater around 1925, with the majority of increases starting around 1935. As a result, moratoriums were placed on well development in the 1970s and 80s. Individual spikes in water rights acquisitions of surface and groundwater were associated with periodic droughts and high crop prices. Change point time series analysis identified three major periods of acquisition of waters rights: 1881–1911 for surface water, 1935–1981 for the unconfined aquifer, and 1945–1967 for the confined aquifer. Collateral effects on stream flows and agricultural acreage also were evident. The annual stream outflows from the SLV declined by $400{\mathrm{hm}}^3$ after 1924, being attributed to surface water extractions. An additional reduction in outflow of $60{\mathrm{hm}}^3$ from 1925 to 1963 was attributed to groundwater extraction. An increase in agricultural acreage by 80,000 ha during the period 1948–1978 fostered increases in groundwater rights acquisition from 1935–1981.