On the Rain and Drainage - Waters at Rothamsled. .'521 
well as in the records for the first four years ; the extent of 
difference is, however, much h'ss in the later years. It clearlj 
follows from these facts that the errors affecting the calculated 
amount of evaporation from the soil during; the spring and 
autumn months will be greatest in the case of the deeper soils, 
and that the calculations made on the results of the 20-inch 
gauge will most nearly represent the truth. 
In the next table the average monthly drainage from the three 
gauges is given as percentages of the rainfall ; in the same table 
will be found the percentage relation of the assumed normal 
drainage (see Table XXllI., p. 319) to the normal rainfall, the 
average of twenty-eight years. 
Table XXIV. — Average Monthly Drainage for 100 Eainfall 
during 10 Years, 1871-80. Also the Drainage for 100 Eain- 
fall with assumed normal quantities of both. 
Observed Drainage for 100 
Normal Drainase for 100 Normal 
Kainfall. 
Rainfall. 
Soil 
Soil 
Soil 
Soil 
Soil 
Soil 
20 inches 
40 inches 
60 inches 
20 inches 
40 inches 
60 inches 
deep. 
deep. 
dei'p. 
deep. 
deep. 
deep. 
January .. 
71 
6 
81 
9 
72-4 
69 
3 
80-4 
70-2 
February 
66 
7 
73 
1 
65-6 
59 
5 
67-4 
58-2 
Blare li 
33 
8 
42 
3 
3G-6 
37 
7 
45-7 
40-3 
April 
33 
8 
38 
4 
35-5 
20 
9 
26-4 
23-0 
Mav 
19 
0 
22 
6 
19-5 
22 
6 
26-0 
231 
Juno 
19 
6 
20 
1 
18 3 
12 
6 
13-2 
11-2 
July .. .. 
27 
1 
28 
0 
24-5 
11 
6 
12-6 
8-4 
August . . 
25 
0 
24 
8 
22-8 
24 
1 
23-8 
21-8 
September 
o7 
5 
33 
4 
29-7 
26 
0 
212 
16-7 
October . . 
53 
6 
53 
2 
44-7 
52 
5 
52-1 
43-4 
November 
69 
7 
72 
4 
64-G 
60 
1 
63-7 
.53-3 
December 
75 
3 
81 
^ 
73-9 
72 
4 
79-5 
70-8 
Whole Year . . 
44 
6 
47 
4 
42-1 
38 
41-6 
35-7 
These figures will require no explanation. The average per- 
centages of drainage to rainfall during summer and winter have 
been already given (page 313). The mean annual proportion 
of drainage to rainfall there given is not exactly the same as 
in the above table, the former results being the mean of ten 
drainage-years, and the latter of ten civil years. 
If we may assume a comparatively constant annual evapora- 
tion from the surface of the soil, it becomes possible to calculate 
approximately the percentage of drainage to rainfall for any 
given rainfall distinctly exceeding the amount of evaporation. 
Thus assuming 17'5 inches as the annual amount of evapora- 
tion, then with a rainfall of 28'3 inches (the present average at 
Rothamsted), the drainage will amount to about 38 per cent. 
