Probably the single most important commodity in the desert was, and is, water.  Water for drinking, washing, bathing, and irrigating crops at the mission was taken out of the Santa Cruz River nearly a mile upstream to the south.  In the 1700s more water flowed and flowed for longer distances on the Santa Cruz.  During the 1900s the water table dropped sharply due to groundwater pumping, drought and other factors.  During the mission period, a water supply system was needed to transport the water from the river to the mission.

The central element of this system was an acequia or irrigation ditch.  This ditch was dug by hand to a depth of about four feet and a width of about five feet.  The exact date the acequia was constructed is unknown, but it was in use in the early 1800s.  The mission acequia has not been used since 1848 when the mission was abandoned.  Some sections of the acequia on private property, however, were used for irrigation as late as the 1960s.

A second element of the supply system was a compuerta (shown in the picture above) - a weir or diversion box - constructed of burnt adobe which was used to direct the water out of the acequia and on to the orchard and fields where all the grains, fruits and vegetables eaten at the mission were grown.  Another use of the compuerta was as a lavadora or a place for washing clothes and dishes, and possibly even bathing.  And lastly, it was used as a holding tank for filling ollas, or earthen jars, that were then carried over a hundred yards to the front of the church, where they were emptied into the two large, rectangular cisterns.

Two rectangular shaped cisterns were built in the front of the church about the same time as the present church. What is visible today are restored cisterns.  A brick-lined ditch, now protected with a covering of earth, leads from the cisterns to the courtyeard on the east side of the church.