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Understanding the surface hydrology of low lying sugarcane fields for a basis of optimised surface drainage criteria

Surface hydrological characteristics of three low lying sugarcane fields in the lower Herbert Valley, North Queensland, were measured and it was found that the regional drainage system controlled the runoff from the field. Low relief slopes and non-integrated drainage design, coupled with high intensity and high volume of rainfall restrict the free movement of water from low lying fields. It was also found that contrary to expectations, inundation of sugarcane fields was not a major cause of yield loss. The recession time for the measured runoff events was between 1.2 and 7.6 hours, well below the critical 72 hours as required for yield loss. The surface water balance of the field shows that the water furrow acts as a preferential pathway for runoff. The surface water balance reveals that, on average only 36% of the runoff from the field exits by the rows. The majority of the runoff (64%) leaves the field via water furrows. 
 
Waterlogging of sugarcane in the heavy textured, far levee soils is caused by the low saturated hydraulic conductivity of the lower soil horizons. This combined with the low specific yield of the soil means that only 100 mm of rain is required to completely saturate the uppermost 1 m of soil. The main study site (Main Palmas Site) was waterlogged for most of the study period (1999-2000). 
 
Evapotranspiration and surface runoff were the significant terms in the annual water balance. Deep drainage contributed more to the annual field water balance than interflow. Sugarcane yields are affected by the presence of shallow watertables. The sugarcane yield components of the 1998-99 crop revealed a strong declining trend across the field bed that was closely related to the depth of the watertable. 
 
It was found that the drainage system behaved such that the flow in the main field flume could be simulated by assuming the water flow from the sugarcane field was being restricted by a high resistance to flow (i.e. Manning’s n). This approach was tested over two wet seasons with 16 separate runoff events and it was found that the model SWMM could be used to simulate the dynamics of runoff from the field. 
 
The Youngs watertable model simulated the dynamics of the shallow fluctuating watertable over a period of 2 years of relatively high rainfall. The Rudd and Chardon model predicted the yield of sugarcane well, the predicted yield were the same or very close to the observed yield for three wet seasons, under conditions of excess soil water. Raising the critical watertable depth in the Gayle model from 0.45 m to 0.4 m permitted a reliable predictor of biomass yield of sugarcane. 
 
Optimised surface drainage criteria were established, namely: 
• The effect of the trash blanket is to slow the removal of water from the fields however as the drainage water is removed from the field within the 3 day limit, therefore retention of the trash blanket is recommended. 
• The upper range of field slopes (0.125%-0.5%) should be such that water is retained on the field to minimise the risk of inundation downstream. To minimise any effects of the high rates of water draining into the regional drainage system, the field slopes should be within the range of 0.01 to 0.125%. 
• An optimum field length appears to be between 300 m and 600 m. this range of field lengths is an optimum between minimising the risk of inundation at a field scale and at the same time, reducing the risk of exacerbating the downstream flooding by decreasing the drainage water flow rate to the regional drainage system. • Water furrows should be retained in fields in areas of high risk of inundation to minimise the effect of inundation. 
• Water furrows should be removed from fields in other areas and the additional area planted to sugarcane.