How much irrigation water flows back into our groundwater?

NIWA is providing three very different types of specialised instrument system to help illuminate the ‘dark side’ of groundwater management.

The instruments will measure the amount, and quality, of irrigation water that finds its way back into our groundwater, partially recharging it. These measurements will deepen our understanding of the factors that influence groundwater recharge and lead to more effective management of agriculture. This need has initiated a multi-agency-funded study to measure this and other aspects of the Canterbury water cycle.

Measuring the amount of water leaving the soil (water out)

We need to know the dynamics of ‘water in/water out’.

Evapotranspiration (ET) is the most useful and significant indicator that we currently use to measure water movement for agriculture. ET refers to two processes: water loss from surfaces such as soil (evaporation) and water loss from the leaves of crops (transpiration).

The weather affects ET; solar radiation, air temperature, air humidity, and wind speed. But ET also depends on the nature of the crop, how it changes with time, and soil moisture levels.

Evaporation and transpiration occur simultaneously and dependently, so are treated as one process. When a crop is first planted, the soil is exposed to the sun and most of the water loss initially is through evaporation. But as the crop grows and starts to shade the soil, evaporation decreases and transpiration increases.

We can estimate ET from meteorological data, but a far more direct, accurate and defensible measurement uses the ‘Eddy Covariance’ (EC) method. NIWA has installed one EC instrument tower and we’ll soon install a second.
Instruments on the EC tower continuously and rapidly sample the turbulent airflow that transports water vapour, heat and carbon dioxide used, or given off, by plants during photosynthesis or respiration.

This method is accurate, but complex, and to get good results, we pay detailed attention to specifics.

The EC tower instruments

The EC tower uses the following sensors to telemeter several parameters needed to accurately calculate ET:

• a sonic anemometer to rapidly and continuously measure turbulence in the air
• a sensor/analyser to continuously measure the concentration of carbon dioxide and water vapour in the air next to the anemometer
• a net radiometer to measure the difference between incoming solar radiation and radiation reflected back to the sky
• a quantum radiometer to measure only that part of solar radiation that the plants use for photosynthesis, when they use carbon dioxide and water to grow
• a pyranometer to measure radiation from sun and sky
• an air temperature/humidity sensor a heat flux sensor to measure heat flow into and out of the ground.

Measuring the amount of water passing into the soil (water in)

We measure the water passing through the soil with lysimeters, located near the EC tower.

Sites need careful preparation and require a significant amount of excavation to create two underground bunkers where the measurement equipment is housed.

We use a containment vessel, or barrel, to isolate a cylindrical core of the crop along with the soil it grows in. We reposition this, with the lysimeter opening at ground surface level, so it looks like and shares the same environment as the rest of the crop, in this case pasture.

We measure the amount of water leaching through the contained soil by piping it out the bottom of the barrel to a rain-gauge located below it in the underground bunker. We can then compare the amount of water that flows through the lysimeter (water out) with the rainfall or irrigation (water in) that we measure in another rain-gauge at the same location.

We use three lysimeters at each site. In one of the three soil cores, we’ve installed three soil moisture sensors at different depths within the barrel, to give a continuous representative vertical soil-moisture profile. Each lysimeter also has a neutron probe access tube to allow regular assessment of soil moisture over the whole soil profile.

To measure water quality we collect the water that passes through the lysimeters and test it for water-soluble nitrates, ammonia and other nutrients.

Teaching a local weather station new tricks

The third type of instrument system we’re using is the NIWA Compact Weather Station (CWS). This gives us a unique opportunity to correlate EC tower ET with that calculated from weather station data and the water balance of the lysimeter.

Operating EC towers is expensive so it’s not viable to put one in every paddock. Neither is it necessary. A practical compromise is to ‘teach’ a local CWS how to estimate ET better, as a CWS is relatively inexpensive to run.

Operating an EC tower near a CWS over a period of time and a wide range of conditions will enable us to ‘fine tune’ the ET equation for local conditions and later estimate a more accurate value of ET from weather data only.

Then, for a specific location, all we would need is a permanent weather station that is ‘taught’ the specific characteristics for that locality to give a vastly improved estimate of ET.

Where to from here?

Currently we plan to install lysimeters at five more irrigated sites. The recently-installed EC tower at Methven will remain in place long term. Soon we’ll install a portable EC tower at Wakanui, the next site. We’ll move this to other sites and use the data to ‘fine-tune’ the CWS ET equation for each. Information from these instruments will also be used to help calibrate ET estimates from weather stations.

Eventually, modest networks of instruments installed throughout Canterbury should enable us to obtain much-needed information on the quantity and quality of irrigation-sourced groundwater recharge and help add substance to information affecting resource consent decisions. An added bonus is that it will provide useful information about local climate change throughout Canterbury.

Acknowledgements

The following agencies are collaborating in the groundwater studies:
Environment Canterbury (ECan), National Institute of Water and Atmospheric Research (NIWA), Aqualinc Research, AgResearch, Crop and Food, Landcare, HydroServices and the farmers who have made land available for instrument installations and provided information on pasture management practices.