This climate drives a highly variable streamflow regime that contributes to the complexity of water management in the Basin. The average annual precipitation over two thirds of the Basin is between 7 inches and 15 inches. It exceeds 25 inches only in the high mountain areas. The Basin lies in a transitional zone between the Gulf and Pacific rainfall provinces. The Basin s complex meteorological conditions are further complicated by the presence of the surrounding mountains.
Storms vary in type and magnitude with the seasons.
Intense isolated thunderstorms are frequent in the summer months. In the winter heavy snow accumulates in the mountains of Northern New Mexico and Southern Colorado. The greatest flood-producing storms have occurred in the transitional seasons, March through May and September through October, when greater temperature differences between air masses moving simultaneously into the region cause increased instability. Miscellaneous smaller sources include a few transmountain ditches in Colorado and discharges of municipal wastewater. The majority of the Basin s surface water comes from melting snow.