A soil-drainage study in southwestern Fresno County

This study describes the conditions associated with a developing drainage problem in southwestern Fresno County, where irrigation water is to be furnished from the joint state and federal San Luis Project.

In a 7½-square-mile study area southeast of the town of Cantua Creek, where no drainage problem has been suspected until recently, a semiperched water table has risen into the crop-root zone. This water is perched on an extensive layer of heavy-textured sediments 100 feet below ground surface. The thickness and limited permeability of this layer have resulted in the accumulation of considerable depths of perched water. The perched water in the study area is part of a groundwater ridge that extends at least 10 miles south of that area and is maintained by deep percolation from irrigation, even though irrigation has been greatly limited by dwindling groundwater supplies. The soluble-salt content of this water has built up the salinity of the surface soils where the water table has risen into the root zone.

Scattered stratigraphic and water-table observations indicate that similar drainage problems could develop over a broad area of southwestern Fresno County when irrigation is intensified with imported water.