Shaping land to improve drainage

Dan Bloomer

LandWISE has been using GPS mapping and modelling to create very accurate contour plans. Some of the software we use can model the effects of rain storms and identify water flow paths, depth and velocity, and areas where water will pond. The flow-path analysis also highlights areas at risk of soil erosion. We can design new land topographies to ensure good drainage and automatically create cut-and-fill maps for GPS-controlled earthmoving.

We used the technology in Horowhenua, helping cropping farmers understand flows, drainage, ponding and soil-sediment loss through erosion. We anticipated a lot of land shaping would be needed to address crop loss from ponding, and to ensure well-managed drainage.

In most cases, little work was needed. Small changes would allow water to flow out of crop furrows, reducing losses, making access much easier and reducing the risk of soil loss when rain storms caused blow outs. By lowering the downstream headland most paddocks would drain adequately.

Of particular interest was the impact row direction can have on drainage and ponding, and on the risk of soil erosion. The modelling showed rows running across the contour increased ponding and increased the severity of erosion. This goes against historic opinion.

When rows run down the contour they can drain freely, with each carrying only the water that falls on it. When rows go across the contour, each is a dam. This stops drainage and causes ponding and crop loss.

When rows run down the contour they can drain freely, with each carrying only the water that falls on it. When rows go across the contour, each is a dam. This stops drainage and causes ponding and crop loss.

During big rain events the dams overflow, and all the water escapes at a single place with great velocity. As row after row is breached, depth and velocity increase, scouring the soil and creating gullies through the paddock. The topsoil ends up in drains, rivers and lakes – exactly where we don’t want it.

We are also using the technology at our MicroFarm in Hastings where we have grown onions for the last three years. Part of an Onions New Zealand Sustainable Farming Fund project to map onion crop variability, this experience highlighted a probable impact of poor surface drainage especially at establishment time. We know we have areas that under-perform because we have made yield maps that show huge variation (0-100+ tonnes per hectare).

How can we calculate the cost of poor drainage? Is the cost of levelling justified?

We used the concept of Yield Gap to estimate crop loss, contractor rates to assess surface levelling costs and created a simple cost-benefit budget.

Yield Gap is the difference between what we could have produced and what we actually got. Our approach was to subtract the achieved yield in any area from the average yield in the top producing area.

We used Arc GIS to create a Yield Gap Map that showed about 8% of the paddock with an average Yield Gap of 44t/ha, 12% lost 28t/ha and 31% 16t/ha. A further 33% of the paddock lost 8t/ha. In total this suggested a crop loss of about 9.7t, or 7.16t/ha. At a return of $450/t, that equates to $3150/ha. Because we knew inadequate irrigation impacted one area, we adjusted the areas to compensate.

The paddock was surveyed with Trimble FMX RTK-GPS and the data exported to OptiSurface® software to create a topographic map, assess drainage patterns, design alternative finished surface options and generate a cut and fill map.

The cut and fill map can exported again and loaded into the FMX system. Then when levelling is done, the FMX system automatically controls scoop or levelling blade height to achieve the design surface.

Our chosen design requires 408 cubic metres/ha of soil to be shifted with a maximum cut of 350mm and maximum fill of 172mm. A cut of 350mm requires that top soil be removed to a temporary storage area, the subsoil levelled and top soil returned. Including ripping to remove compaction caused by the levelling machinery, and as we are dealing with a small area, we budgeted a contractor cost of $2000/ha.

An onion loss of $3150/ha and a drainage cost of $2000/ha indicates a return on investment in the first year of $1150/ha. We should pay for levelling with our first crop.

  • Dan Bloomer is a consultant for LandWISE Hastings



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