Modelling mid-span water table depth and drainage discharge dynamics using DRAINMOD 6.1 in a sugarcane field in Pongola, South Africa

Abstract

Determining optimal subsurface drainage design parameters through monitoring of water table depth (WTD) and drainage discharge (DD) at various combinations of drain depth and spacing is expensive, both in terms of time and money. Thus, drainage design simulation models provide for a simplistic and cost-effective method of determining the most appropriate subsurface drainage design parameters. In this study, the performance of the DRAINMOD model (Version 6.1) in predicting WTDs and DDs was investigated for a 32 ha sugarcane field in Pongola, South Africa. Water table depths were monitored in 1.7 m deep piezometers installed mid-way between two drains by using an electronic dip meter with a beeper, while DDs were measured at drain lateral outlet points, using a bucket and a stop watch. Both WTDs and DDs were monitored from September 2011 to February 2012. Results of the DRAINMOD model evaluation in predicting WTD, during calibration period, showed that there was a very strong agreement between simulated and observed WTDs with a goodness-of-fit (R²) of 0.826 and a mean absolute error (MAE) of 5.3 cm. Similarly, simulated and observed DDs during the model validation period also showed very strong agreement, with an R² value of 0.801 and an MAE of 0.2 mm∙day^-1. Results of simulated WTDs at various combinations of drain depth and spacing indicated that in clay soil a WTD of 1.0 to 1.5 m from the soil surface can be achieved by installing drain pipes at drain spacing ranging from 25 to 40 m and drain depth between 1.4 and 1.8 m. On the other hand, in clay-loam soil, the same 1.0 to 1.5 m WTD can be achieved when the drain pipes are installed at drain depths ranging from 1.4 to 1.8 m and corresponding drain spacing ranging from 55 to 70 m. Based on these results, it was concluded that DRAINMOD 6.1 can reliably be used as a subsurface drainage design tool in the Pongola region. This would simplify the design of subsurface drainage systems and the formulation of subsurface drainage design criteria for different crops and soil types found in the area and possibly throughout South Africa.

Keywords: drain depth; drain spacing; Hooghoudt’s equation; model performance; saturated hydraulic conductivity; steady state conditions