A diagnosis of sub-surface water table dynamics in low hydraulic conductivity soils in the sugar cane fields of Pongola, South Africa Mphatso Malota a, b , Aidan Senzanje b, * a Department of Water Resources Management and Development, Mzuzu University, P/Bag 201, Luwinga, Mzuzu, Malawi b School of Engineering, University of KwaZulu-Natal, P/Bag X01, Scottsville, Pietermaritzburg, South Africa article info Article history: Received 24 March 2015 Received in revised form 25 September 2015 Accepted 1 October 2015 Available online xxx Keywords: Drainage design Drain depth and spacing Soil salinity Sum of water table exceedance Water table depth abstract Water and land are the two natural resources restraining crop production in South Africa. With the increasing demand for food, emphasis has shifted from the sole reliance on rain fed crop production, to irrigation. The deterioration in irrigation water quality from surface water sources is, however, posing a big challenge to the sustainability of irrigated crop production. This is because more water is required for leaching, resulting in shallow water tables in agricultural lands. The installation of well designed sub- surface drainage systems alone is not enough; the provision of timely maintenance is also necessary. In this study, the extent and severity of problems as a consequence of shallow water tables and their possible causes were investigated at three sugarcane fields in Pongola, South Africa, having low hydraulic conductivity soils. Also investigated were soil salinity levels and the temporal variation in the salinity of the irrigation water. A water table map of a 32 ha sugarcane field was generated, using observed water table depth (WTD) data from 36 piezometers monitored from September 2011 to February 2012. Out of the total 32 ha under cultivation, 12% was found to be affected by shallow WTDs of less than the 1.0 m design WTD. The inability of natural drainage to cope with subsurface drainage needs and the poor maintenance of subsurface drainage systems contributed to the shallow water tables in the area. Furthermore, the currently adopted drainage design criteria also proved unsatisfactory with mean observed water table depth and drainage discharge (DD) of 20% and 50%, respectively, less than their respective design levels. The salinity of the irrigation water was, on average, 32% higher than threshold tolerance level of sugarcane. The root zone soil salinity levels at the three study sites were greater than the 1.7 dS m 1 threshold for sugar cane. The subsurface drainage design criteria adopted at the site needs to be revisited by ensuring that the slope of the land is taken into consideration in the drainage design in addition to adhering to a recommended maintenance schedule. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Globally, the area affected by soil salinization and waterlogging is increasing at approximately 1.5 million ha per year (Armour and Viljoen, 2008). Soil salinization and waterlogging have resulted in the failure of many irrigation schemes to achieve their targeted yield projections (Patil et al.,1982; Gupta and Yadav, 1993; Sinha et al., 1991; Kool, 1993; Wolde Kirkos and Chawla, 1994; Dandekar and Chougule, 2010). Despite irrigation and drainage being two inextricable agricultural water management systems (Singh et al., 1999; Hurst et al., 2004; Bahceci et al., 2006; Hirekhan et al., 2007; Graciana and Nkambule, 2012), more often than not, irrigation systems worldwide have been developed without proper consideration of the need for drainage improvement. According to Vandersypen et al. (2007), this is chiefly because more funds are channelled to irrigation development, while the agricultural drainage sector receives little or no financial support. Contrary to the increasing worldwide acknowledgement of the links between food security, irrigation and subsurface drainage improvement, most of the world's irrigated lands are poorly drained. For instance, out of the total 270 million ha irrigated worldwide, only 22.2% is provided with appropriate drainage (Schultz et al., 1999) with Africa and Asia accounting for 90% of the poorly-drained land (Wood, 2008). Unfortunately, these two con- tinents are also the most affected by hunger and food insecurity (Armour and Viljoen, 2008). In Africa alone, 80 million ha are re- ported to be waterlogged and salinized (Tana, 2008). This is despite the need to increase the contribution of irrigation to the total world * Corresponding author. E-mail addresses: senzanjea@ukzn.ac.za, Senzanje@gmail.com (A. Senzanje). Contents lists available at ScienceDirect Physics and Chemistry of the Earth journal homepage: www.elsevier.com/locate/pce http://dx.doi.org/10.1016/j.pce.2015.10.004 1474-7065/© 2015 Elsevier Ltd. All rights reserved. Physics and Chemistry of the Earth xxx (2015) 1e9 Please cite this article in press as: Malota, M., Senzanje, A., A diagnosis of sub-surface water table dynamics in low hydraulic conductivity soils in the sugar cane fields of Pongola, South Africa, Physics and Chemistryof the Earth (2015), http://dx.doi.org/10.1016/j.pce.2015.10.004