We have been making elevation maps of cropping paddocks. It is the first step in determining the optimum plan to manage drainage and sediment loss.

If it can, water will flow downhill. It flows faster on steeper, smoother land and faster when it is deeper. The faster it flows, the higher the risk of soil erosion and loss.  If water can’t find a down-slope, it will sit until it soaks or evaporates away. If that takes too long, plants will suffer.

The rate at which water infiltrates (soaks into) the soil is largely related to soil texture and structural condition. A coarse soil generally lets water in faster than a “tight” clay soil. If the natural porosity has been damaged by over-cultivation, water movement is reduced. Other factors such as hydrophobic organic matter (think oilskin raincoats) also stop or reduce infiltration.

Step two is using a computer to analyse our detailed elevation maps and determine where water will pond and how deep it will be. Knowing how much land is affected lets us gauge the cost of ponding. We can “apply” a rain event and see where the water runs, how deep and how fast. This lets us identify, and make plans to manage, erosion risk areas.

The third step is creating plans to remove water in a controlled way; not too fast and not too slow. For many years this has been done by laser-grading. This creates a flat plane with a set slope so water can drain at a set rate across the land. Unfortunately it often requires a huge amount of soil movement.

High accuracy GPS and smart software has enormous benefits. Our detailed elevation maps are analysed to create optimised cut-and-fill plans that move only enough soil to enable surface drainage. We don’t mind that the surface is not a flat plane, so long as the water can move. We can set the maximum slope (to avoid erosion) and the minimum slope (to avoid ponding) and the direction we want water to flow.

OptiSurface software calculates where to cut high points and fill low points, balancing the soil so one just fits the other. Rather than a flat plane, we get a varied terrain with lower points connected to ensure drainage.  While much is automated, there is still a need for farmer and designer input. The theoretical design must be practical.

Once completed, design files are loaded back into the tractor which controls the height of a levelling blade to get the exact cuts and fills we have determined. Advice is to use the same type of GPS for both surveying and levelling. Subtle differences between brands and even models of GPS system can create problems. It is also important to have a base station within 2 km so the vertical error is minimised. Having 40 or 50mm of GPS error when the soil cut is 20mm just doesn’t work.

The series of maps cover a case study 40ha cropping property. We mapped it using the farmer’s Trimble FmX and analysed the data in OptiSurface.

Map 1 shows the existing topography on the farm, each colour change being a 1m change in elevation.

Map 2 shows where water will pond, and how deep it will get.

Next we looked at some solutions.

Map 3 shows the possible topography if part was levelled to a single plane (purple through green strips) and part was OptiSurfaced (green through red strips).

Map 4 shows the cut and fill required to achieve each solution. The amount of movement for a single plane is huge – over 700m3/ha for single plane and under 20m3/ha for OptiSurfaced areas.

Map 5 shows the cut and fill required if the whole area is optimally resurfaced.

A new initiative in Horowhenua will reduce sediment and nutrient input into the Arawhata Stream and Lake Horowhenua. And it will increase farm productivity by managing drainage and reducing crop losses.

LandWISE is working with local farmers, the Tararua Vegetable Growers’ Association and Horizons Regional Council in this Ministry for the Environment supported project.

The team will create integrated drainage and sediment control plans for up to 500ha of cropping farms. The plans will identify ways to manage risks using mix of land shaping and storm water management, supported as necessary by erosion control and sediment capture techniques. Where appropriate, cut and fill plans for reshaping will be prepared, enabling farmers to have automatic control of earthmoving equipment.

The current drainage system, actually the legacy of an historic stock drinking water race scheme, cannot contain run-off from severe storm events. Inadequate drains spill flood waters on to cropping land, creating strong rivers that erode cultivated soil and wash it, along with crops, into the Arawhata and into Lake Horowhenua. Nobody wins.

Local grower, John Clarke, believes additional drainage he has installed has addressed a large part of the problem. Interceptor drains and increased capacity capture and contain water from higher up the catchment, and guide it safely to the outlets.

More is needed. The whole system must work together from top to bottom. A problem on one farm inevitable flows on to the next.
Horizons staff have completed a survey of the existing drainage system, measuring channel dimensions and culvert sizes. They met with local growers to hear first-hand of the issues the growers understand only too well. They are now designing a new system that will be the core of enhanced drainage in the catchment.

On-farm, precision surveying with GPS tractors has begun. Using their Trimble technology, the growers can map their properties in 3D, with an error of millimetres.

The data collected will be processed using OptiSurface software that determines ponding areas, flow paths and depths. It can create optimized cut and fill plans requiring the minimum amount of soil movement that allows effective drainage. Those plans are fed back into the tractor guidance system and control the blade depth on ground shaping equipment.

Expectations are that ponding areas will be identified and removed through strategic levelling. This removes two problems: the bathtubs of ponded, stagnant water that can collect and row ends and destroy crops, and the risk of blow-outs that cause erosion and sediment being lost to the lake.

The third level of sediment management is retaining even small amounts of sediment through use of sediment control structures and filter plantings along farm drains. Small but continuous losses add up over time and can constitute a significant loss of nutrients from the farm, as well as more sediment load into the lake.

The project will see individual farm plans for each property that can be integrated in New Zealand GAP and used to demonstrate good practice to stakeholders.

The project is one of eight that together form the Fresh Start for Fresh Water Lake Horowhenua Freshwater Clean-up Fund programme. Horizons’ Fresh Water and Science Manager, Jon Roygard notes efforts to restore the lake have been ongoing for several decades, including in 1987 stopping the discharge of raw sewage into the lake. Recently, and almost complete, a full native planting buffer strip has been established around the lake.

Other efforts include harvesting lake-weed to remove nutrients, a sediment trap where the Arawhata enters the lake, storm water treatment upgrades, a boat wash facility, a fish pass, some further riparian fencing and planting of the tributaries and work with Dairy farmers to complete farm plans.