Irrigation efficiency under Northwester storm conditions

Abstract

This project aimed to identify and model the effects of Northwesterly storms on irrigation uniformity and efficiency. In doing this, climate conditions associated with Northwesterly storms were also investigated. Historical climate data from one representative site on the Canterbury Plains was used to establish a definition for a Northwesterly storm event. This was a maximum daily temperature greater than 25°C, wind gusts higher than 40 km/h and relative humidities less than 30%. Northwesterly storms generally occur in summer months. ASCE standardised reference evapotranspiration was calculated for six sites on the Canterbury Plains for 2010. The results show an obvious seasonal variation; summer months have higher evapotranspiration values due to increased solar radiation. Location was also found to be important; evapotranspiration values increase from south to north and west to east. The highest daily evapotranspiration values occurred at Waipara station, which is located the farthest north of the six sites and is in a sheltered valley. To determine the impact of key meteorological variables (relative humidity, air temperature and wind speed) on reference evapotranspiration, a sensitivity analysis was performed for each of the six sites. Evapotranspiration was relatively insensitive to air temperature in winter, and achieved a maximum in summer. Sensitivity coefficients associated with air temperature varied spatially, with evapotranspiration further north being more responsive to changes in temperature. However, changes in relative humidity were found to have the greatest impact on evapotranspiration, and no relationship was observed with wind speed coefficients. Two statistically derived equations (Yazar’s & Bavi’s models) were used to calculate the evaporative losses throughout 2010 at the Winchmore EWS site. The results from Yazar’s model showed generally lower losses than Bavi’s model. However Yazar’s model was more impacted by the Northwesterly storm events showing evaporative losses of up to 9% during 2010. Their models did not take account of wind effects, which would further increase this loss. If Yazar’s model is correct, alternative irrigation schedules would be useful during Northwesterly storm events.