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The variability of winter drainage in England and Wales
Author(s) -
Rose D.A.
Publication year - 1991
Publication title -
soil use and management
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.709
H-Index - 81
eISSN - 1475-2743
pISSN - 0266-0032
DOI - 10.1111/j.1475-2743.1991.tb00859.x
Subject(s) - drainage , cockle , return period , hydrology (agriculture) , physical geography , environmental science , mathematics , statistics , geography , geology , ecology , archaeology , biology , geotechnical engineering , flood myth
. Estimates of winter drainage, D , are available for 82 locations in England and Wales for each winter during the 35‐year period, 1940/1 to 1974/5. At each location, D is approximately normally distributed. The variability, expressed as a coefficient of variation, is approximately inversely proportional to the average drainage; it is large, ranging from 20–30% in Wales and the uplands of northern and western England to 60–70% in eastern England. Assuming a normal distribution, the probability of exceeding a given amount of drainage is easily found. The proportion, f of solute initially in the soil that is leached each winter can be calculated using these estimates of D in a simple layer model. However, f is not normally distributed, but can be fitted by the Type‐1 extreme‐value distribution. If there are n years of data, and m is the rank of a value in order of decreasing size, the estimation of the probability, P , of exceeding the m th largest value, f m , is P ( f f m ) = m /( n +1), and the return period, t , is 1/ P years. Using the transform, W = In[—In P ], graphs of/against W yield families of lines that, for a given location and depth in the soil, depend only on soil type, and from which extreme values likely to be encountered in a given return period can be forecast. Results are discussed for Mildenhall, Suffolk (= 99 ± 68 mm), Cockle Park, Northumberland (±= 275 ± 125 mm), Welshpool, Powis (±= 373 ± 121 mm), Carmarthen, Dyfed (±= 764 ± 196 mm) and Ambleside, Cumbria (±= 1280 ± 276 mm).