Paddock Topography Survey and Interpretation

In January 2015 Page Bloomer Associates surveyed the MicroFarm using Trimble RTK equipment from GPS Control Systems and a mobile base station lent to us by Drumpeel Farms.

GPS Control Systems Trimble antenna mounted on Scout used forhigh accuracy paddock survey
GPS Control Systems Trimble antenna mounted on car used for high accuracy paddock survey

The RTK survey equipment provides high accuracy GPS with better than 20mm horizontal accuracy 95% of the time. The vertical accuracy is within about 30mm. Over the period of our survey, the point to point accuracy is likely to be very good.

View of the GPS Control Systems Trimble FMX screen showing the trail of points recorded during the paddock topographical survey
View of the GPS Control Systems Trimble FMX screen showing the trail of points recorded during the paddock topographical survey

To make sense of the topographic survey, the data were processed using OptiSurface Surface Flow modelling software. OptiSurface accepts data from Trimble GPS equipment and will create a very detailed topographic map.

The power of OptiSurface is in its surface water flow analysis, and subsequently in its ability to calculate minimum soil movement required for water to drain naturally from the site.

NOTE: We have displayed the OptiSurface maps on top of a Google Earth image of the MicroFarm. Unfortunately this Google image is slightly transposed to the west, so the four Zones marked are shown about half a zone too far to the right.

Topography of Onion PAddock: Colours 100mm elevation bands, contour lines 10mm elevation change
Topography of Onion PAddock: Colours 100mm elevation bands, contour lines 10mm elevation change

This image shows the topography of the paddocks in which onions have been planted. The contour interval is only 10mm, with each colour band representing 100mm of elevation. The total elevation change shown in this map is less than 500mm.

A locally high spot (yellow) is seen just to the left of Zone 1. A locally low spot (pale blue) can be seen just right of Zone 2. Zone 3 is on an even slope. Zone 4 is in the lowest part of the whole paddock (purple).

OptiSurface analysis Surface ponding expectation map of onion paddock with 100mm high beds
OptiSurface analysis Surface ponding expectation map of onion paddock with 100mm high beds

This map shows the depth of ponding that can be expected after heavy rain events. This analysis is for crop beds 100mm high running the length of the paddock (left to right). The depth is the depth of water from the base of the furrows.

Brown areas will have less than 10mm of ponding. Dark purple areas will have 80mm or more of ponding, more than enough to impact crop if it persists. Zone 1 is in a better drained area, Zone 2 in a potentially damper area and Zone 3  moderately well draining. Zone 4 is in a potentially wet part of the block with ponding of around 80mm depth likely in furrows after heavy rain.

We shall watch and note.

 

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