On-farm sampling density and correction requirements for soil moisture determination in irrigated heavy clay soils in the Gezira, central Sudan

Abstract

Using the neutron scattering technique, with separate calibration for each measuring depth and temperature corrections, an over-sampling experiment with a worst case analysis was conducted in tenant irrigated fields under arid conditions. The purpose was to better understand actual on-farm soil moisture distribution as well as to determine minimum sampling density requirements for water use efficiency calculations in the heavy cracking clay soils of the Gezira irrigation scheme, central Sudan, under inhomogeneous watering conditions. Results show that actual soil moisture inhomogeneities can seriously distort the moisture distribution and water use pictures if the sampling density is too low. In a 2.1 ha end field under Gezira conditions 20 equally spaced neutron probe samples had to be collected from the 30 cm soil depth if the total experimental errors were to be kept within 12.5% of the average moisture content being measured. Sampling density requirements increased to 24, 28 and 33 samples for worst case error limits of 10%, 7.5% and 5% at 30 cm depth. At the agronomically more important lower depths, at or below 70 cm, less than 10 samples only could be afforded with an error of 10% at 70 cm, of 15% at 50 cm and of 20% at 30 cm, the errors typically becoming smaller at larger depths throughout. Credible soil water averages were obtained with this sampling. Field moisture patterns were well recognized when averaging several days of measurements.

Introduction

In line with the call for better understanding of agrodiversity to improve its management, there is an increasing attention for the actual variations of soil properties in cropped soils and the way African farmers may exploit these variations (Steenhuijsen Piters, 1995). Analysis of such cases requires other approaches in research to be further developed in order to characterize, measure and understand relevant variations in agroecosystems in general (Almekinders et al., 1995). The interest for such precision farming is also rising in western agriculture. Sampling methodologies for such studies have to be further developed.

In the Traditional Techniques of Microclimate Improvement Project (TTMI/AN, 1996), traditional cropping techniques are compared with (other) current methods. Tenants in the Gezira scheme, central Sudan, abandoned labour intensive irrigation methods for unattended irrigation that is suspected to waste water. The difference in water use efficiencies between these methods had to be quantified on-farm (Ibrahim et al., 1999).

Little quantitative attention has been given to soil moisture heterogeneities in general and in the irrigated soils of the Gezira in particular. Results from the on-farm over-sampling experiment presented in this paper are intended to contribute to knowledge on sampling density requirements as well as moisture variations in these and other swelling clays. This should lead to better understanding of irrigation water management in farmers' fields, especially in surface irrigation methods which are known for their low application efficiencies (FAO, 1971). Soil-physical studies that have attempted to develop flow theory for swelling soils have not reached an operational stage for field applications (Bouma and Loveday, 1988).