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Journal of Hydrology (NZ) 47 (2): 67-84 2008 © New Zealand Hydrological Society (2008) Hydrological impacts of irrigated agriculture in the Manuherikia catchment, Otago, New Zealand Stefan W. Kienzle1 and Jochen Schmidt2 1 Department of Geography, University of Lethbridge, Alberta, Canada 2 National Institute of Water and Atmospheric Research, P.O. Box 8602, Christchurch, New Zealand. Corresponding author: [email protected] Abstract to losses in the conveyance system, on­ Water for irrigation is becoming an in­ farm application losses, increased actual creasingly critical component of New evapotranspiration, and the development of Zealand’s rural economy. Projections of a deeper rooting system compared to natural expanding agricultural sectors indicate vegetation. increased demands for water and a reliable water supply. Hence, it is pertinent to assess Introduction the impacts of water demand for current Water for irrigation is becoming an and future irrigated agriculture on catch­ increasingly critical component of New ment hydrology. This case study focuses Zealand’s rural economy. More intensive on estimating the hydrological impacts of farming systems are usually accompanied by irrigated agriculture in the Manuherikia a demand for increased water quantity and catchment upstream of Alexandra, Central a reliable water supply. Projections indicate Otago, New Zealand. In order to assess that New Zealand’s dairy, horticulture the impacts of irrigation on streamflow, and viticulture sectors will all expand in five land­use scenarios, including three the future, and it follows that there will be irrigation scenarios with varying efficiencies, growing demands for water for irrigated are investigated with the ACRU agro­ agriculture (Ministry of Agriculture and hydrological modeling system (Agricultural Forestry, 2004; Parliamentary Commissioner Catchments Research Unit (ACRU), for the Environment, 2004; Doak, 2005). Department of Agricultural Engineering, The economic implications of these University of KwaZulu­Natal, Republic of predicted future trends have been assessed South Africa, http://www.beeh.unp.ac.za/ (Ministry of Agriculture and Forestry, 2004; acru/). The results show a 37% loss of mean Doak, 2005), but little is known about the annual water yield under current conditions impacts of irrigated agriculture on catchment due to inefficient irrigation practices. Even hydrology and water resources due to increased with the most water­efficient irrigation water demands relative to a static, and infrastructure, this loss could be reduced by under future climate conditions potentially only 20% – meaning a 30% loss of mean declining, supply. As water resources become annual water yield. These results emphasize scarcer and water supply becomes less certain the significant and inevitable water demands due to forecasted climate change (IPCC, and catchment water yield losses associated 2007), it is pertinent to assess the impacts of with irrigated agriculture, which are due intensified irrigation practices on catchment 67 hydrology. The report ‘Growing for good’ This case study focuses on estimating the of the Parliamentary Commissioner for the hydrological impacts of irrigated agriculture Environment (2004) contains some qualitative in the Manuherikia catchment upstream of estimates about current trends in water Alexandra, Central Otago, New Zealand. quality and water quantity for New Zealand’s The aim is to highlight differences in the regions. These qualitative estimates need to catchment’s hydrological responses under be translated into quantitative evaluations natural conditions and under modified to manage water use in New Zealand’s conditions associated with irrigated agri­ catchments and to balance the economic culture. This is achieved by simulating both value of irrigation with environmental costs the natural hydrology of the catchment and sustainable agricultural practices (Poff and a number of irrigation scenarios using et al., 2003). Potential impacts of irrigation on a physically­based model of catchment water resources include changes to river flow hydrology. A suitably structured hydrological rates, in particular low flows, and lowering of simulation model, operating at appropriately groundwater levels as a result of abstraction sensitive time steps and spatial scales, is and changes in recharge rates. Surface water required. The daily time step, physical­ and groundwater systems sustain complex conceptual and multipurpose ACRU agro­ ecosystems. The change of water flow rates hydrological model (the acronym ACRU is and storage quantities may have adverse derived from the Agricultural Catchments effects on those ecosystems, potentially Research Unit, Department of Agricultural altering them significantly (Larned et al., Engineering, University of KwaZulu­Natal, 2007; Parliamentary Commissioner for the Republic of South Africa; Schulze, 1995), Environment, 2004; Poff et al., 2003). was selected. In addition to calculating In an ideal setting, the evaluation of the all elements of streamflow, it can simulate impacts of land­use change, including the reservoir yield, irrigation supply/demand introduction of irrigated agriculture, large and return flows and has been structured reservoirs, irrigation canal systems (races), explicitly to represent processes of land­use farm dams and inter­basin transfer, would change impacts. In this paper we describe the characteristics of the Manuherikia catchment, be based on long­term streamflow obser­ and the configuration for use with the vations both upstream and downstream ACRU model; we apply the model with a of such a development. However, in New baseline land cover to simulate pre­settlement Zealand, available streamflow records are conditions and show the effects of several often not long enough nor dense enough scenarios of irrigated and non­irrigated to allow the quantitative assessment of the agriculture on the Manuherikia’s water impact of irrigated agriculture. Therefore, as resources. an alternative approach, the streamflow can be simulated for pre­ and post development scenarios. Such a simulation requires that Study area the selected model be not only able to The Manuherikia catchment is located simulate the major elements and processes north­east of Alexandra, Central Otago, New of the hydrological cycle, but it also needs Zealand (Fig. 1). The Manuherikia River to be sensitive to land cover changes and is a tributary to the Clutha River – one of incorporate elements of the infrastructure for the largest rivers in New Zealand (the largest irrigated agriculture likely to effect catchment in terms of flow volume). The Manuherikia hydrology. catchment has an area of 3035 km2 at 68 Figure 1 – Map of study area: Numbers 1 to 4 are the locations of the four gauging stations (see Table 2 for details). Alexandra. The central valley bottoms of the and associated potential evapotranspiration catchment, divided into two major valleys, ranges within the catchment are largely due constitute one of the largest intra­montane to the wide variations in altitude. The valleys depressions of the tilted fault mountain and in the Manuherikia catchment are sheltered basin systems of Central Otago, and are from south­westerly and north­westerly filled with Tertiary and Pleistocene deposits. rains and have the lowest recorded rainfall The northernmost headwaters of the in New Zealand. Rainfall increases from catchment reach an elevation of 2100 m and an average of 330 mm/y around Alexandra drop 1200 m over a distance of 20 km to the to 1500 mm/y in the northern Hawkdun headwater valley bottoms. The central valley Range (Fig. 2b). Rainfall occurs throughout bottoms of the catchment have an altitude of the year, with approximately 60% falling in 100–500 m (Fig. 2a). spring and summer. In the valleys only 3% Due to the distance to the sea and the of annual precipitation falls as snow, while high altitude in Central Otago, the climate on the highest ridges snowfall can constitute is the most continental in New Zealand. up to approximately a third of the annual Temperatures range from a maximum of precipitation. 35°C in summer to a winter minimum of The Manuherikia catchment can be –20°C. The annual mean temperature is divided into two major subcatchments approximately 10°C. Temperature ranges (Fig. 1). The eastern Ida Valley drains the 69 eastern and south­eastern Otago uplands to 1500 m, soils are dominantly high country (‘Rough Ridge’), which has a lower rainfall yellow­brown earths, with silver tussock, than the northern part of the catchment hard tussock and snow tussock making up (Fig. 2b). The western Manuherikia Valley the main vegetation. Below 900 m, fescue is separated from the Ida Valley by the and blue tussock grassland dominates. Soils central Raggedy Range, where the Idaburn between 700 and 900 m are mainly recent River drains through a single gorge into the alluvial soils, while the soils in the central Manuherikia River. A five year flow record valley bottoms below 700 m are yellow­grey from gauging station 75252 Poolburn at earths (Ahlers and Hunter, 1989). Know­ Cobb Cottage indicates very low streamflow ledge of the associated soil textures (Fig. 2e) contributions from the Idaburn River, with a and soil depths are essential for hydrological mean annual flow of 1.32 m3s–1, constituting simulations as they govern the hydrological a runoff coefficient of approx. 9% under behaviour. current conditions. This is an indication Land use in the Manuherikia catchment of high evapotranspiration

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