20 ' FORREST SHREVE 



in., which raised the moisture at 3 cm. on the 14th to 17 %, but 

 did not affect the lower soil, which was still dry, under the in- 

 fluence of the rainless period, having a moisture of 11 % at 15 

 cm. and 12 % at 30 cm. During the eight days from August 



14 to August 22 there were five rainy days with falls of the fol- 

 lowing amounts: 0.52 in., 0.46 in., 0.53 in., 0.76 in., and 0.88 in. 

 The total effect of such an unusually wet week was to raise the 

 moisture to 27 %, 28 % and 32 % at the three depths, reading 

 from the surface. These figures undoubtedly come close to rep- 

 resenting the maximum conditions of soil moisture for the Tum- 

 amoc clay.^ 



It will be observed that the light rains between December 23 

 and February 6 — 0.06 in., 0.09 in., 0.07 in., and 0.02 in. respec- 

 tively — were without effect upon soil moisture, except in so far 

 as they contributed to a lowering of the evaporation rate and a 

 consequent conservation of the soil water. Rains of 0.10 in. on 

 October 14 and December 20 were without effect on the soil 

 moisture at 15 'cm. and 30 cm. A rain of 0.15 in. on November 

 26 had the apparent effect of raising the moisture at 15 cm. from 

 13 % to 16 %, but this rise was in large measure due to the rain 

 of 0.70 in. on November 13, which had already caused a rise of 

 the 15 cm. soil from 11 % to 13 %. A rainfall of 0.18 in. on Feb- 

 ruary 28 did not prevent a continuation of the drop in the mois- 

 ture at 15 cm. from 18 % to 16 %, although the rain of 0.13 in. 

 on March 6 was responsible for a rise in moisture at 15 cm. three 

 weeks later, but possibly onlj^ in conjunction with the rain of 

 0.18 in. The rain of 0.28 in. on April 2 caused a marked rise in 

 the well desiccated surface soil, but effected only slight rises at 



15 cm. and 0.30 om. depth. 



It is possible to say in summation, that rains of less than 0.10 

 in. are without effect on the moisture of the soil at 15 cm., at 

 least if such fall occurs after a dry period of two or three weeks, 

 on a soil with a moisture of 4 % to 7 % at 3 cm. A rain of 0.15 

 in. may have an effect if it falls on soil which is alreadj^ moist, 



3 The highest moisture recorded by Livingston (as cited) was 35 % at 30 cm., 

 after two weeks with a rainfall of 18.52 in. The maximum water-retaining power 

 of Tumamoc clay was determined bj' Livingston as 48 % of its dr}^ weight. 



