2 nd World Irrigation Forum (WIF2) 6-8November 2016, Chiang Mai, Thailand W.1.1.03 1 DETERMINING THE OPTIMAL IRRIGATION STRATEGY FOR ROTATIONAL GRAZING SYSTEMS Birendra K.C. 1 , Magdy Mohssen 2 , Henry Wai Chau 3 , Andrew Curtis 4 , Richard H. Cuenca 5 , John Bright 6 , MS Srinivasan 7 And Keith Cameron 8 ABSTRACT The soil moisture content start and stop points (the irrigation trigger points), for rotationally grazed pasture are often ambiguous and imprecise. A water balance model, IrriCalc, was used to analyse the impacts of different soil moisture triggers upon the irrigation required and subsequent drainage losses over a long-term period to investigate optimal irrigation ranges within the soil water holding capacity. The analysis took into consideration rainfall and evapotranspiration uncertainties. For the analysis, available daily climatic data over a 15 year period (2000 to 2015) was used to account for climatic variability.The experiments were conducted at the Lincoln University Dairy Farm (LUDF), Canterbury, New Zealand during August 2014 to March 2016.The results showed a trigger point to start irrigation at 55 and 60% of plant available water (PAW), respectively on the shoulder (September to October and March to April) and peak (November to February) irrigation seasons, and stopping irrigation correspondingly at 80 and 90% of PAW were optimal. Adopting this irrigation strategy will help better manage environmental risk, caused by nutrient leaching loss through increased drainage, and production risk resulting from soil moisture stress. Maximising effective rainfall during the irrigation season as well as minimising drainage will help irrigatorsbetter balancingthe growing tension between water use for agricultural production and the environment. Keywords: Threshold soil moisture content, Rotationally grazed pasture, IrriCalc, Optimal irrigation range, Irrigation and drainage, New Zealand. 1. INTRODUCTION Settled agriculture started about 10,000 years ago and farmers have been practicing controlled irrigation for over 6,000 years (Postel,1999). However, despite having several ways to measure soil-water-plant-atmosphere parameters, there is still a lack of irrigation scheduling that addresses changing weather and crop water demands. There has been an increasing awareness to vary the amount of water applied based 1 PhD Candidate, Department of Environmental Management, Lincoln University, PO Box 85084, Lincoln 7647,Christchurch, New Zealand;E: Birendra.K.C@lincolnuni.ac.nz 2 Senior Lecturer, Department of Environmental Management, Lincoln University, PO Box 85084, Lincoln 7647, Christchurch, New Zealand;E: Magdy.Mohssen@lincoln.ac.nz 3 Lecturer, Department of Soil and Physical Sciences, Lincoln University, PO Box 85084, Lincoln 7647, Christchurch, New Zealand; E: Henry.Chau@lincoln.ac.nz 4 Executive Chief, Irrigation New Zealand, PO Box 69119, Lincoln 7640, Christchurch New Zealand; E: acurtis@irrigationnz.co.nz 5 Professor Emeritus, Department of Biological and Ecological Engineering,116 Gilmore Hall, Oregon State University, Corvallis, USA; E: Richard.Cuenca@oregonstate.edu 6 Director, Aqualinc Research Limited, Christchurch, New Zealand;E: j.bright@aqualinc.co.nz 7 Hydrologist, National Institute of Water and Atmospheric Research Limited, Christchurch, New Zealand; E: MS.Srinivasan@niwa.co.nz 8 Head, Centre for Soil and Environmental Research, Lincoln University, PO Box 85084, Lincoln 7647, Christchurch, New Zealand;E: Keith.Cameron@lincoln.ac.nz