excess of this amount was considered available 

 to the alfalfa plants. 



For each year, 1964 through 1968, the total 

 amount of water available for plant growth was 

 calculated for each plot in the experiment. 

 The starting point was the available water in 

 the soil after spring snowmelt. To this was 

 added the precipitation recorded at the site for 

 the remainder of the growing season for each 

 year. Water from contributing areas was added 

 to their respective benches. Runoff (if any) 

 from natural slopes was subtracted from the 



water available for plots on slopes. The rela- 

 tionship of total water available to alfalfa 

 yields indicates that 73 percent of the varia- 

 tion in yield is associated with variation in 

 the amount of water available (fig. 6). 12 The 

 economic value of the alfalfa, indicated in the 

 right-hand scale, was obtained by applying a 

 price of $17 per ton. Alfalfa production and 

 its economic value evidently increases rapidly 

 as total water increases, particularly at the 

 lower water levels. The value of each addi- 

 tional inch of water, calculated from the re- 

 gression formula is as follows: 







And the added 





Then the alfalfa resulting 



(or marginal) 



If the water 



from one more inch of 



product would 



available is: 



water would be: 



be worth: 



Inches 



Tons per acre 



Dollars 



10 



0.287 



4.88 



12 



.263 



4.47 



14 



.239 



4.06 



15 



.229 



3.86 



16 



.215 



3.66 



18 



.191 



3.25 



20 



.167 



2.84 



22 



.143 



2.43 



24 



.119 



2.02 



26 



.095 



1.62 



28 



.071 



1.21 



30 



.047 



.80 



32 



.023 



.39 



34 











The average water available for the na- 

 tural slopes in the years included in the study 

 was about 14 inches. At this level, an addi- 

 tional inch of water would produce 0.239 tons 

 of additional alfalfa, which would be worth 

 $4.06. The additional returns for a second inch 

 would be $3.86, and for a third inch, $3.66, or 

 $11.58 for three additional inches. 



Table 7 shows that producing 100 acres of 

 alfalfa on a 4-percent slope with 42-foot 

 benches and no contributing areas would cost 

 $3,231, while on a natural slope the cost would 

 be $2,099. The level bench system would 

 therefore cost $1,132 more than on the natural 

 slope, or the equivalent of $11.32 per acre. 



To recover the additional cost, it would be 

 necessary to produce additional alfalfa worth 

 at least $11.32. In the previous paragraph, 

 it was shown that three additional inches of 

 water would result in alfalfa production worth 

 $11.58, or slightly more than enough to cover 

 the additional costs. Thus, this particular 

 system of benches would appear to be profit- 

 able for alfalfa production if it could be de- 

 pended upon to trap and store the equivalent of 



1 2 Results of the regression analysis, significant at 

 the 0.001 probability level, were: 



Y = -2.05131+ 0.40764W - 0.00601 W 2 

 R2= 0.7290 

 where Y = alfalfa yield 



W = water available. 



12 



