The study gives a systematic methodology to determine field hydraulic conductivity using Guelph Permeameter (GP), its limitation based on the experiences gained in the field and a comparison of its results with that estimated from the predictive method for the soils in Central Bihar. Predictive methods applying empirical formulae are used to ascertain hydraulic conductivity from easily determined factors like bulk density, porosity and water content.
Physically based hydrological models, soil water balance, groundwater flow and transport models require values of saturated and unsaturated hydraulic conductivity to solve the sub-surface flow and transport equations. Several field, laboratory and predictive methods are available for determination of this parameter. However, these methods have some advantages and some limitations. GP is used as a tool to determine the field hydraulic conductivity even when the water table is low. It is portable, durable and allows rapid field calculation of saturated and unsaturated hydraulic conductivity.
GP method is not suitable for soils exhibiting strong heterogeneity, particularly in a vertical direction within the depth of water standing in the hole. If the bottom of the hole contains two distinct layers or is intercepted by macropores such as rat holes or root channels etc., the saturated hydraulic conductivity will be negative, which is physically unrealistic.
The study puts forth the following recommendations –
- Extensive field studies by GP are necessary in different soil types particularly in heavy textured soils.
- Hydraulic conductivity may be calculated in the field itself and if found unrealistic, the experiment needs to be repeated in the close proximity at the same depth.
- Setting the GP at different depths in the same location and taking average of the hydraulic conductivity obtained at each depth may be representative of the profile. It is advisable to study the soil survey report of the area or study the soil profile by auguring before using GP.
- Soil cores of the same location and depths may be brought in the laboratory and the hydraulic conductivity can be determined by using ICW Permeameter. This result obtained by GP can be verified with the one obtained from predictive methods. This will provide an in-depth understanding of the problems and reliability of GP.
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