A study of spatial and temporal biotic changes in an agricultural drainage canal
Eleven years ago, the California State Legislature authorized the construction of a peripheral drainage canal for the San Joaquin Valley. Observations suggest that drainage water, which ultimately will come from the entire valley, may have adverse effects on water quality in San Francisco Bay and the surrounding delta, into which it will drain. The valley is a vast agricultural area, so the proposed "master drain" would discharge predominantly agricultural waste water into the bay. This has stimulated agricultural waste water studies, but comprehensive data on conditions in agricultural drainage canals are lacking. Most of the published and unpublished information on agricultural runoff that is available was produced by state and federal agencies. There are marked differences in the quality of irrigation water applied to the land and the quality of the residual water draining from the land (Sylvester and Seabloom, 1963). Studies describing changes in physical and chemical factors during irrigation are numerous (Knight, 1969; U. S. Department of Agriculture, 1969; Williams and Wadleigh, 1968; Bullard, 1966; Sylvester and Seabloom, 1963; Reid, 1961; Flippin, 1945). Factors mentioned most frequently in runoff situations are sediment, temperature, oxygen content, inorganic plant nutrients (such as nitrogen and phosphorus), and pesticides (California Department of Water Resources, 1968; Mackenthun and Ingram, 1967). Other sources concerning physical and biotic runoff relationships will be listed in the pertinent sections of the following text. Biotic studies concerning agricultural drainage canals are less numerous. The U. S. Public Health Service (1960) and Vinyard (unpublished) have studied the biota of drainage water, but their work dealt mostly with phytoplankton ecology. Aside from general references on pollution and its effect on organisms, data concerning the total biota, and its possible regulation by waste water conditions, are lacking. Recognizing the need for more information on agricultural runoff, I undertook a study of Main Drain Canal, which carries agricultural drainage. The purpose of the study was twofold: 1) to document the spatial and temporal biotic changes in a drainage canal, and 2) to demonstrate, when possible, whether or not these changes were due to irrigation effluents. The remainder of this paper deals with this analysis of Main Drain Canal and its implications for future drainage projects.