The San Augustin Plains or watershed is a closed basin 1,993 square miles in size. Closed basin means that there is no outlet or surface stream draining the basin. Therefore, the watershed gets no water from outside sources. There are no perennial streams within the basin. The only water it receives is from rain and snow that falls directly on the watershed. The water that is in the basin is considered ancestral water mainly from the past Pleistocene ice age. The recent water additions in the last couple of centuries has been less than evapotranspiration rate and the leakage rate along fractures and faults, as water has not accumulated to form a lake as it did in the past Pleistocene Era. That large ancient lake has long since evaporated.

Two counties share the watershed'Catron County's portion is approximately 1,551 square miles in size, a little over three-quarters of the watershed, while Socorro County is about 441 square miles in size. Land ownership within the basin is approximately as follows: Bureau of Land Management (BLM) 222.4 square miles, Forest Service (FS) 438.8 square miles, private land 830.7 square miles, and state land 500.1 square miles. The actual plains portion of the watershed is approximately 603 square miles in size or about 30% of the watershed. The Blodgett and Titus report of 1973 at the time of their study concluded that the basin was receiving approximately 100,000 acre-feet of water per year from rain and snow; however the basin seemed to be losing that same amount from evapotranspiration and leakage. They assumed that the main source of leakage was along the southern boundary. Since the time of that study New Mexico has been in an extended drought and probably not receiving anywhere near that amount of water annually. The New Mexico state geologic map shows many faults that cross the watershed boundary and more recent geologic mapping have identified even more faults.

The San Augustin Plains is a topographically high area, which means that groundwater in the basin will seek to flow in all directions away from the basin. The gradient flows from the basin have been calculated by a report done by R. G. Myers, J.T. Everheart, and C. A. Wilson in 1994 titled G