ESTIMATION OF SOIL HYDROLOGICAL CHARACTERISTICS USING STRUCTURE AND TEXTURE

INTRODUCTION

It is possible to derive rough estimates of the hydraulic conductivity (K) and the drainable pore space (μ) from observations of the soil profile. This is because these soil hydraulic qualities depend on soil texture and structure. Table A1.1 average presents μ values, compiled by FAO (1980) and based on data from the USBR (1984), together with K values estimated from the μ/K relationship.For soils with distinct horizontal layers, the vertical K may be taken as being at least 10 and on average 16 times lower than the horizontal one.As these estimates may be imprecise, more realistic K values are obtained through field measurements, as described in Annex 3.However, interpreting the soil structures mentioned in Table A1.1 may not be easy. It should be done through observations of soil profiles, but shallow groundwater levels often prevent excavation of soil pits. Moreover, soil texture and structure should be evaluated when the soil is moist throughout.However, in special cases, it is possible to estimate drain spacings directly from the visual aspects of the soil profile, as was done by people with detailed local experience in the Zuiderzee polders, the Netherlands, where it was the only possible method – drain spacings of 8, 12, 16, 24, 36 and 48 m were distinguished and the choice between possibilities was possible.For pure sands (almost without clay and silt), an estimate is:where:K = permeability (m/d).m50 = median size of grains above 50 μm. Half of the weight is above this size, half below.