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Water and Nitrogen movement under horticultural land

by Steve Green, Plant and Food Research

Clean water and healthy soils are essential components of modern agriculture. The challenge to orchardists and farmers is to maintain the benefits of irrigation and agrichemical use while minimising any adverse effects on the environment. International sustainability standards for water and agrichemicals are being developed and orchardists will increasingly need to show that their practices are sustainable. They will need tools to monitor, audit and predict the effect of land use activity to minimise their environmental footprint.

Three good reasons

It is difficult to measure unsaturated or vadose zone water and nutrient losses from the root zone for three main reasons. First, vadose zone flow rates span many orders of magnitude. Second, the distribution of water fluxes is often very variable over short distances. Third, the placement of water flux sensors can disrupt the flow causing either convergent or divergent flow with resulting inaccuracies in estimates. At present, there is no standard method for measuring soil water flux.

This article provides a brief description of a passive wick drainage flux meter that has been developed at Plant and Food Research to measure the drainage fluxes focusing on horticultural land. The article also presents a measurement and modelling comparison where nitrate leaching is calculated using Plant and Food’s soil plant atmosphere system model. These comparisons, along with some fine-tuning, are very important to provide growers, industry and regulators with confidence in the quality of the data as well as the results.

Flux meter design

Scientists at Plant and Food Research have built large number of passive wick lysimeters to monitor water and nutrient losses under productive land. The basic device consists of a convergence tube, a funnel, a hanging wick and a subterranean reservoir. A hanging water column is created, and drainage water is pulled out of the lysimeter while the lower soil boundary is passively maintained at a pressure below atmospheric pressure so the soil stays unsaturated. The degree of unsaturation depends upon the wick length, the flux rate and the soil type.

The drainage flux can be measured automatically with a tipping spoon device that collects water draining from the glass fibre wick. The wick passively controls the pressure head in the soil at a value of minus 60 cm, approximately equal to the length of the wick, which is close to field capacity in most soils. A control tube is placed directly on top of the wick to minimise any divergent or convergent flow. The control tube can contain either an intact or a re-packed soil column. The device is usually buried well below the root zone depth. Drainage water is collected in a subterranean reservoir where it can be extracted and analysed for nutrients and co-contaminants.

Plant and Food Research has installed many drainage flux meters over the past six years at a number of experimental sites around New Zealand. Here we report results from three of these sites, in the Hawkes Bay. At each location direct comparisons were made between drainage data from the fluxmeters and model outputs from the soil plant atmosphere system model.

Model framework

Plant and Food Research’s model considers water, solutes such as nitrogen and phosphorus, pesticides, heavy metals, dissolved organic matter, as well as microbial transport through a one-dimensional soil profile. The calculations run on a daily time step.

The soil water balance is calculated by considering the rainfall and irrigation going in, and the losses − plant uptake, evaporation, run-off and drainage − of water from the soil profile. The model includes components to predict the carbon and nitrogen budget of the soil. These allow for a calculation of plant growth and uptake of nitrogen, various exchange and transformation processes that occur in the soil and aerial environment, recycling of nutrients and organic material to the soil biomass, and the addition of surface applied fertiliser or effluent.

Orchards in Hawkes Bay

Field sites were established on kiwifruit, apple and peach orchards in Hawkes Bay as part of a Sustainable Farming Fund project. The kiwifruit block was located close to Bridge Pa on free draining Esk Sand. The peach block was located in a research orchard near Havelock North on a Heretaunga silt loam. The apple orchard was located at Mr Apple’s Close Orchard near Whakatu on a Hastings silt loam. Irrigation to the apple trees and kiwifruit vines was with micro-sprinklers at a rate of 10 to 12 mm once every two to three days. Irrigation to the peach trees was with drippers at about half the daily rate.

Six non-recording drainage flux meters were installed at each site, along with a weather station to record the local micro-climate. Arrays of time domain reflectometry probes were installed in the root zone soil to a depth of two metres to monitor changes in soil water contents.

Photosynthetically active radiation sensors were located on the orchard floor to measure light interception and deduce changes in the green leaf area over the growing season. Sap flow sensors were also installed in the apple and peach trees to relate the actual water use to the daily potential evapo-transpiration.

Measurements and modelling results

The main aim of these field measurements was to quantify the soil water balance and to find appropriate values for all of the model’s parameters. A measurement and model comparison for the kiwifruit orchard is shown in the graph. There was agreement between the measurements and model results of soil water content and drainage losses under the kiwifruit orchard. In this case the experimental data was used to optimise the crop factor, KC, that relates actual water use of the vines to the potential evapo-transpiration rate.

During the first two months of the experiment the root zone of the kiwifruit vines remained below field capacity. This was despite regular irrigation aimed at minimising any water deficit over the period between flowering and fruit set. A large amount of drainage of around 100 mm occurred in early summer, despite there being little rainfall during December and January.

This was probably the result of applying too much irrigation early in the season to the free draining sand. After this, the grower’s strategy was to withhold irrigation for a period of about six weeks before harvest in an effort to improve the dry matter content of the fruit. The soil water content slowly declined and all drainage stopped shortly after irrigation was suspended. Drainage did not occur again until the middle of June, following more than 200 mm of winter rainfall.

Drainage from the apple orchard was similar to, but slightly less than, drainage from the kiwifruit. Drainage from the drip irrigated peach orchard was much lower, about 25 per cent compared with apple and kiwifruit, and it was delayed by about one to two months.

All drainage water was collected and analysed for nitrate nitrogen. The apple and peach orchards leached between 10 and 18 kg per hectare of nitrate nitrogen from the root-zone soil, over the winter and spring period. This is perhaps the first time that drainage rates and nitrate leaching losses have been quantified under tree crops in Hawke’s Bay.

Summary

In the past it has been difficult to obtain reliable measurements of unsaturated water and nutrient fluxes. The passive wick flux meters are now available which offer a cost-effective and reliable way of obtaining such measurements.

Reliable measurement techniques are needed to improve our understanding and to test our models. There is no standard method of measuring water and nutrient fluxes, yet drainage flux meters appear to be suitable across the board. Observation, experiment and modelling are the essential components in putting together scientific information, developing appropriate and realistic predictions, and guiding our response to complex problems. Model measurement comparisons, along with fine tuning, are very important to provide growers, industry and regulators with confidence in the quality of the data and of the models.

Steve Green works at Plant and Food Research in Palmerston North