Sources of Water Flow
A major effort of the baseline study was to identify the sources of summertime flow in the lower Salinas River. There are three major point sources of discharge within the study area; the two City of Salinas treatment plants and the Blanco Drain, which discharges an average of about 0.11 m3 per second (4 cfs) approximately 8 km. (5 mi.) upstream from the mouth. This drain discharges agricultural irrigation water runoff from about 2,400 ha. (6,000 ac.) north of the river. In addition to these major point sources, there are numerous nonpoint sources of agricultural return water along both sides of the river and along the north side of the lagoon.
There are two other potential sources of water to the lagoon: groundwater inflow from the surrounding land, and seawater inflow from Monterey Bay. The magnitude of the groundwater inflow (or outflow) was measured by constructing groundwater wells around the periphery of the lagoon and calculating the magnitude and direction of flow based on soil permeability and water level difference in the wells. Seawater inflow was estimated by performing a salt balance on the lagoon water, and by a previous study which directly measured seawater inflow (Muckel etal . 1964).
Since it was impossible to measure the nonpoint flow into the river directly, it was calculated by performing a mass balance between the various sampling points on the river. The average nonpoint inflow between stations was estimated as the difference between the upstream flow and any known point source inflows within the reach. An average value for this flow per unit length of river was then computed and applied to the entire length of river and lagoon to give a total value for nonpoint inflow.
Using the data thus collected, an overall water budget for the river and lagoon was constructed. This is presented in figure 2. Surface flow into the lagoon accounted for about 92% of the inflow; of this, about 45% was from the treatment plants and the rest was from the Blanco Drain outfall and nonpoint sources. In the river, the quantity of flow from the treatment plants ranged from 100% in the upper reaches of the study area to about 45% at the head of the lagoon. Groundwater inflow from the periphery of the lagoon was negligible; seawater inflow was also very small. One source of water which was
Figure 2.
Low-flow water balance for the lower Salinas River and Lagoon.
not measured was subsurface flow in the river channel.