Location: Moonie, Qld, Australia

Field Area: 133 ha or  333 acres

Irrigation Method: No Irrigation. Dryland/Rainfed with no furrows

Crops: Wheat, Cotton, Sorghum/Milo

This field naturally has many small 'gilgais' or craters which are typically 30m (90ft) wide and 0.3m (1ft) deep. The gilgais severely limits surface drainage which makes this field marginal for cropping. Ponding Analysis (see image below) based on surveyed topography shows 14% of the field or (17.5 hectares or 44 acres) experiences standing water. This creates water logging problems for crops in these areas but also delays timely machine operations for the rest of the field waiting for the ponded areas to dry out. Also the steeper edges of the gilgais approached 5% slope. This increases runoff into the bottom of the gilgais and reduces infiltration other areas of the field, therefore reducing the crop yield of the field even further.

Yield Loss From Water Logged Areas
Say an average yield reduction of 25% over 14% (17.5ha) of the field with a gross income of US$2000/ha from cotton and wheat rotation  = US$8,750/yr

Yield Loss From Less Timeliness of Planting and Harvest
Say an average yield reduction of 5% over 115.5ha of the field with a gross income of US$2000/ha from cotton and wheat rotation = US$11,500/yr

Yield Loss From Reduced Infiltration Uniformity
Say an average yield reduction of 5% over 115.5ha of the field with a gross income of US$2000/ha from cotton and wheat rotation = US$11,500/yr

Estimated Total Cost Of Surface Drainage Problems = US$32,000/yr

An Optimum Surface Landform design was generated for the field with a minimum grade of 0.05% to promote drainage. A maximum grade of 1% and a maximum rate of change of 0.05% per metre was also applied to promote more uniform infiltration of rainfall.  See the Existing Topography, Prosposed Topography and Proposed Cut Fill Maps below.

Earthworks
Volume of Cut: 7,500 m³ or 9,900 yd³
Volume Of Fill:  6,300 m³ or 8,300 yd³
Average Cut per Area: 56m³/ha  or 30 yd³/acre!!
Cost: US$22,500 assuming an earthmoving rate of US$3/m³

Survey and Design
133ha at $25/ha = US$3325

Total Cost Of Surface Drainage Solution: US$26,000

Payback Period: $26,000/$32,000 = 0.8 years. Less than one year!

Proposed Topography

Cut Fill Map