Understanding water losses from irrigated pastures on loess-derived hillslopes

Abstract

Irrigation is likely to increase water losses from hillslopes, particularly on loess-derived soils with impeded drainage. This is important as irrigation of these soils in New Zealand is increasing. A field site was established to measure runoff from a pasture hillslope irrigated by a centre-pivot in South Canterbury. Between November and March, 161 and 199 mm of irrigation was applied, with 23% more at the bottom of the slope. Runoff varied with position in the hillslope, with 3.5 times from the bottom plot (52 mm) compared to the top. Over the length of the slope (40 m) this represents a potential loss of 9% of precipitation, or 21% of the irrigation. Evidence for saturation excess and infiltration excess runoff was observed, with antecedent soil moisture conditions being a key factor. Pasture production and water use efficiency (WUE) also varied with slope, the least (4.6 t DM/ha or 12 kg DM/ha/mm) observed at middle and most at the top of the slope (10.1 t DM/ha or 23 kg DM/ha/mm). This was likely due to a combination of differences in radiation and soil conditions. There was indication that pasture growth was limited by water availability at the top and potentially excess at the bottom of the slope. Our results indicate potential for improving irrigation practices.

Improving irrigation management on hillslope pastures

“…Plant available water capacity (PAWC), defined as the water retained above the fragipan between the matric potential of -1500 kPa and -10 kPa, was estimated at 57 mm, 42 mm and 94 mm for the top, mid, and bottom plots respectively. Detailed soil descriptions are shown in Langer et al (2020). Catchment surface runoff (Catchment) was measured through a weir at the outflow.…”

“…To investigate runoff on the hillslope within the catchment (Hillslope), three 40 m 2 runoff plots were installed in September 2019 along the south facing slope, approximately 65 m away from the catchment outflow weir. Langer et al (2020) provides in-depth information on the runoff plot setup and monitoring. The surface-hydrologically isolated plots were positioned at the top (slope angle of 4º), mid (slope angle of 18º) and bottom (slope angle of 15º) of the south facing slope parallel to the tracks of the centre pivot irrigator.…”

“…For each hillslope, three different areas were simulated, corresponding to the three runoff plots of the field trial. Soil parameters were setup based on measurements and the slope angle of each area (detailed soil descriptions are given in Langer et al (2020)). Values of CN for runoff calculations were gathered from standardised tables (SCS 1985) with minor adjustments based on measurements for the first irrigation season, which resulted in values of 88, 92, and 90 for top, middle and bottom plots.…”

“…A collaborative research programme was established in 2019 to improve the understanding of the effects of irrigation on runoff losses in the loess rolling hill landscape of South Canterbury. Results from the first year showed that water use efficiency varied for the different slope classes (Langer et al, 2020). This suggested that irrigation management could be improved to ensure a better water supply for plants.…”

“…Landscapes with hillslopes are prone to surface and subsurface water runoff (Bircham and Gillingham 1986;Bretherton et al, 2010). There is evidence that irrigating grazed hillslopes with impeded drainage, such as slowly permeable fragipans present in loess sheets in South Canterbury and North Otago, can promote water losses via surface runoff (Laurenson et al, 2018;Langer et al, 2020). However, hydrological understanding of these systems is still limited.…”