31 



protected slopes. 



Canyon-bottom growth is usually water-loving. It may be a 

 meadow, a swamp, alders, willows, the larger sycamores, cotton- 

 woods or cedars, or a mixture of them. Under certain conditions 

 the use of water by vegetation in any selected section of a given 

 canyon may be determined by the difference in stream measurements 

 at its upper and lower boundaries, particular care being taken to 

 force the underflow to the surface at points of measurement. In 

 some instances natural rock barriers to underflow exist and the 

 stream flows naturally over them, but in other cases artificial 

 controls, such as submerged dams, may be necessary to bring the 

 ground water to the surface. Before an investigation is under- 

 taken, the canyon and its surrounding area should be examined to 

 determine the possibility of stream-bed losses through rock fis- 

 sures. Where fissures occur or accurate measurements are impossi- 

 ble, the investigation is not feasible. The possibility of side 

 inflow from canyon walls also should be examined. The Division 

 of Irrigation used the method referred to above in determining 

 the consumptive use of water by canyon-bottom vegetation in south- 

 ern California (see p. 66). (2, 4). 



The diurnal fluctuation of flowing streams is of importance 

 as an indicator of the daily withdrawal of water from the soil by 

 plants. It has long been observed that stream flow decreases by 

 day and recovers by night, the plotted daily discharge curves 

 showing a series of alternate low and high points which occur at 

 approximately the same time each day. The daily decline is the 

 result of the action of plants in withdrawing water from the satu- 

 rated zone, and recovery is due to the nighttime decrease of 

 transpiration. Therefore, as the daily fluctuations of the stream 

 surface depend upon transpiration from plants, the same factors 

 which affect transpiration likewise affect flowing water, but in 

 a reverse order. That is, when bright sunshine, warm weather, or 

 hot winds cause high rates of transpiration the corresponding 

 stream flow will be low: but it will increase in volume when 



