Developing Soil Water Retention Curves to Guide Irrigation Needs for the Soils of Pioneer Farm, Waialua, Oahu

Abstract

Improper management of water is a waste of a scarce and valuable resource as well as posing a threat to the environment. To improve irrigation scheduling in Hawai‘i, a web-based irrigation scheduling tool which incorporates real time weather data and soil moisture data will be implemented to guide farmers with their irrigation management. Soil moisture data is a key component to determine a proper irrigation recommendation, however there is little soil moisture data available for Hawai‘i’s important agricultural soils. In this research, I developed soil water retention curves (SWRC) for three agricultural soils to estimate a soil moisture threshold, which is used to calculate plant available water (PAW) in cropping systems. We collected three soils, two Mollisols (Pulehu and Ewa series) and an Oxisol (Lahaina series) with varying physical properties from the Pioneer Farm in Waialua, Oahu, and developed SWRCs for each soil. We demonstrated that soil texture and bulk density affected soil water retention where the Pulehu soil with a high sand content and the Ewa soil with a high bulk density showed less water retention and a lower amount of PAW compared to soils with a high clay content and pseudosand properties (Lahaina). The soil moisture threshold was -20 kPa for the Pulehu soil, -40 kPa for the Lahaina soil, and -70 kPa for the Ewa soil. Using the threshold, we estimated the time intervals between irrigation events using a generalized potential evapotranspiration rate that was not crop specific. We determined that the Pulehu and Ewa soils retained the least amount of PAW requiring daily irrigation applications. On the other hand, the Lahaina soil with a high clay content and high porosity retained the most PAW and a longer interval between irrigation events.