70 



FORESTRY AND IRRIGATION 



February 



was obtained. The time between plant- 

 ing and harvesting averaged 120 days, 

 and during this time the mean total 

 evaporation amounted to 24.15 inches, 

 or a rate of 20.13 inches per 100 days 

 and a total of 43.08 computed to 214 

 days. On these evaporometers the 

 mean yield of dry matter per acre was 

 13,881 pounds, 10,000 being a large 

 field yield. This observed mean rate 

 of evaporation is, therefore, likely to 

 be some higher than for ordinary field 

 yields under the same climatic condi- 

 tions, and the indications are that the 

 evaporation from the field surface un- 

 der crop may not be quite as large as 

 has been found from the continuously 

 wet soil surface. It may appear im- 

 possible that such a relation as this can 

 exist, but the probability of it being 

 true is made clearer when it is under- 

 stood how great is the reduction in the 

 rate of evaporation from soil surfaces 

 when a thin layer at the top is allowed 

 to become dry, and especially if it is 

 loose and in the condition of an earth 

 mulch. The surface of the soil in the 

 plant evaporometers was maintained in 

 the condition of a good earth mulch 

 three inches deep, and the effectiveness 

 of earth mulches is clearly brought 

 out by the results given in the follow- 

 ing table, where the mean amount of 

 evaporation from firm and mulched 

 surfaces of six soil types are given : 



EVAPORATION FROM Six SOIL TYPES DURING 28 



DAYS WITH SURFACES FIRM AND UNDER THREE- 



IXCH EARTH MULCHES, COMPUTED TO 214 



DAYS SEVEN MONTHS. 



Surface Surface Differ- 

 Soil Types. Firm. 



Inches. 

 Sandhill ............ 14-37 



Selma Silt Loam... 22.13 

 Pocoson ........... 25-64 



Norfolk Sandy Soil. 31.87 

 Goldsboro Compact 



Sandy Loam ...... 40.85 



Norfolk Fine Sandy 



l. im ............. 49.79 



Under Three- ence. 

 inch Mulch. 



Inches. Indies. 



1.57 12.80 



5.92 16.21 



7.ii 18.53 



5.89 25.98 



6.7 3 

 8.7o 



34.12 

 41.09 



These observations were made at 

 Goldsboro, N. C., in June and July, 

 and they show that there is a very 

 great protection against evaporation 

 ;< Horded by three-inch earth mulches; 

 also, that there is a large variation in 

 the loss of water from different soil 



types under like conditions through 

 surface evaporation. As the data of 

 this table were obtained during a com- 

 paratively dry period, and when the 

 temperature was high, the rates and 

 differences are perhaps as" large as are 

 likely to occur, on the average, in the 

 irrigated districts of the West, under 

 such conditions. 



In another series of observations 

 made under what must have been con- 

 ditions closely similar to those of arid 

 climates, columns of capillarily satu- 

 rated soil 10 feet long were maintained 

 under a continuous draught and with- 

 out moistening the surface, during 314 

 days where the temperature ranged be- 

 tween 60 and 90 F. The loss of 

 water which occurred was only deter- 

 mined percentagely, but from the 

 weights of the soils per cubic foot the 

 total evaporation, computed to 214 

 days, must have been very close to the 

 amounts in the next table : 



EVAPORATION FROM TEN FEET IN DEPTH OF Two 

 SOIL TYPES UNDER ARID CONDITIONS CONTINU- 

 OUSLY DURING 314 DAYS AMOUNTS 

 COMPUTED To 214 DAYS. 



Surface Surface Differ- 

 Soil Types. Firm. Under Three- ence. 



inch Mulch. 



Inches. Inches. Inches. 



Sandy Loam 2.85 2.49 .36 



Heavy Clay Loam. . 6.55 4.65 1.90 



From these observations, and those 

 given in the last table, it is clear that 

 whenever the surface of a field is pro- 

 tected by a layer of dry soil, whether 

 this is firm or loose, the evaporation 

 will be relatively small ; but less from 

 the loose than from the firm surface. 



It is probable that the losses of water 

 by evaporation from fields in the irri- 

 gated districts of the western United 

 States ranges all the way between 1.57 

 inches and 50 inches from April i to 

 October 31. If this is the case, the 

 maximum conservation of soil mois- 

 ture, through wind-breaks, is likely to 

 be found to lie somewhere between 40 

 per cent of 1.57 inches and of 50 inch- 

 es, or between .63 inches and 20 inches 

 for the period April i to October 31. 



The wind - breaks themselves, of 

 course, transpire not inconsiderable 





