Turning theory into practicality

The final-year Lincoln University PhD candidate was the 2021 winner of the NZ Society of Soil Science/Fertiliser Association of NZ Postgraduate Bursary Award. The $5,000 award recognises the efforts and likely contribution to New Zealand soil science arising from a doctorate study.

Deuss’ postgraduate research saw her lead a long-term field study on soil and catchment hydrology in Southland. The findings will help understand the role mole and tile drains play in that region’s unique landscape.

“We were interested in understanding the hydrology of a small catchment that is drained by a mole and tile drainage system on a sheep farm near Otahuti in Southland,” she told Rural News.

The Otahuti landscape of gently undulating, loess-derived soils (i.e. soils formed in deposits of wind-blown silt) with low permeability and poor drainage is widespread across the Southland plains – as well as in other parts of NZ.

Deuss explains that mole and tile drainage is a common soil management practice in Southland. It aims to reduce the amount of time that the soil is saturated with water from rain and/or a high water table.

“These drainage systems improve the land for agriculture. However, they also have environmental and hydrological consequences, which challenge the sustainable management of these landscapes,” she adds.

“Several studies have shown that nutrients, faecal bacteria and sediment are discharged from these drains into surface waters. This causes water quality to decline.”

Deuss says improving the understanding of the soil hydrology of catchments with modified drainage is important for the development and calibration of hydrological and nutrient-loss models. These are heavily relied on for managing agricultural landscapes.

“Because mole channel drainage is an unregulated activity, there is very little knowledge about its regional extent, or how long the mole drains actually last,” she explains.

“Commonly they are assumed to degrade after about 10 years. However, there are very few studies that actually confirm this.”

Her research found that by using a non-invasive geophysical technique called Ground-Penetrating Radar (GPR) to identify the mole channels that, even after 30 years without maintenance, these artificial drains are still present and functioning.

“We found that the mole drainage network was not installed at one time, but over multiple times across several years, each with a slightly different configuration. The result has been a very dense network of mole channels.”

Deuss and her team installed a comprehensive network of sensors to monitor how the water moved.

“We discovered that most of the water that exited the tile drain was sourced from the flat, upslope areas between two drainage hollows (swales), and that this water movement was facilitated by the mole channel network.”

They also found that winter and spring were the seasons where most contaminants were lost to surface water, and that nitrate losses were low, but phosphate and pathogen (E. coli and Campylobacter) losses could be very high.

“This finding is important as it helps to target the timing of any on-farm solutions that mitigate surface water contamination from mole and tile-drained loess landscapes,” she says.

“We also discovered that, despite being low permeability soils, the movement of water to groundwater (i.e. deep drainage) was in fact a significant component of the water balance.”

Deuss says this finding is important because deep drainage is commonly assumed to be negligible when modelling flow pathways in these soils.

“This is another piece in the puzzle of understanding how water moves from the surface to become groundwater, so that we can better estimate the current state and quantity of our groundwater resources,” she explains.

“It will also help with predicting changes in hydrology and water quality – due to past and future land use change and the process of climate change using hydrological modelling techniques.”

Deuss believe the results of her research will help to manage the landscape more effectively.

“Our study is a good demonstration of how long-term, integrated studies (soil variability, surface hydrology, soil and groundwater process and water chemistry) can be used to address multiple questions from many different angles and across a variety of interrelated disciplines.”

She says these include things such as planning mitigation strategies, quantifying peak contaminant loads and when they occur, as well as helping calibrate computer models of hydrology of drain catchments for better understanding of stream flow and contaminant loading. And improved soil mapping techniques.

Team Effort!

Kirstin Deuss is quick to acknowledge the many people that helped with her successful research project.

“I couldn’t have done this without the funders of my PhD, Manaaki Whenua Landcare Research and Environment Southland,” she says.

“I‘d also especially like to thank Regan Strang and Rachael Millar of Strang Farms Ltd at Otahuti. They welcomed me and my sensors and probes onto their beautiful farm and worked around me and my solar panel-mounted deer posts for three whole years.”

Deuss says while her PhD has been challenging, it has also been very rewarding. She adds that gaining the award was a real confidence booster before starting her my new career at Manaaki Whenua Landcare Research in February.

“I wouldn’t be where I am without the support of my many great mentors, friends and colleagues, who have given me so much of their time and energy to help turn ideas into reality and put it all into the written word!”