2008 A P P L I E D A N D N A T U R A L S C I E N C E F O U N D A T I O N ANSF JANS Journal of Applied and Natural Science 9 (2): 720 - 729 (2017) Modeling phosphorus distribution under different fertigation strategies in onion (Allium cepa L.) crop Sanjay T. Satpute 1* and Man Singh 2 1 Department of Soil and Water Engineering, Punjab Agricultural University, Ludhiana -141 004 ( Punjab), INDIA 2 Water Technology Centre, Indian Agricultural Research Institute, New Delhi - 110012, INDIA *Corresponding author. E-mail: sanjay4471@gmail.com Received: July 21, 2016; Revised received: December 16, 2016; Accepted: April 13, 2017 Abstract: The understanding of soil and nutrient dynamics under drip fertigation is relevant for crop production as well as water and nutrient management. The aim of this study was to generate information about the distribution of phosphorus (P) under different fertigation strategies for onion production on sandy clay loam soil during 2007 -2008 to 2008-2009. The study involved field experiment, laboratory analysis and modeling of P distribution. The phospho- rus distribution data in the field were collected, analyzed and used to calibrate and validate the solute transport mod- el HYDRUS-2D for sandy clay loam soil. The performance of HYDRUS-2D was evaluated by comparing its simulat- ed values with the observed values of soil moisture and nutrient concentration. The coefficient of determination (R 2 ), root mean square error (RMSE) and mean absolute error (MAE) were used as model performance indicators. The range of R 2 between 0.72-0.99 for water as well as nutrient distribution indicates good correlation between the ob- served and simulated values. The MAE and RMSE values for water and nutrient distribution were in between 0.0009 to 0.0039 which indicated the accuracy of the model. From these results, it can be concluded that the model is per- forming well for predicting the P concentration in the soil as well as the soil moisture distribution for onion crop grown under sandy clay loam. The model was also validated for water and phosphorus distribution with the ob- served values at the end of the crop season and found to be performing well. The HYDRUS-2D model may be used to carry out simulations for different soil types and with different fertigation and irrigation strategies for developing guidelines. Keywords: HYDRUS-2D, Fertigation strategy, Phosphorus distribution, Onion ISSN : 0974-9411 (Print), 2231-5209 (Online) All Rights Reserved © Applied and Natural Science Foundation www.jans.ansfoundation.org INTRODUCTION Drip irrigation is often preferred over other irrigation methods because of the former’s high water-application efficiency on account of reduced losses, surface evaporation and deep percolation. Improved water use efficiency under drip irrigation by reducing percolation and evaporation losses, provides environmentally safer fertilizer application through irrigation water (Mmolawa and Or, 2000). In countries where the cost of water is very low, such as India, the adoption of drip irrigation has initially been very slow. Recently however, drip irrigation combined with fertigation has been found to benefit farmers because of the very high efficiency of fertilizer use for such irrigation schemes. There is an ample scope for improving the efficiency of fertilizer use through fertigation, if the movement and reactions of fertilizers in the soil are well understood. Fertigation enables the application of soluble fertilizers and other chemicals along with irrigation water, uniformly and more efficiently (Patel and Rajput, 2000; Narda and Chawla, 2002). The dynamics of the water within the soil volume surrounding the emitter represents a prerequisite to design irrigation systems as well as to manage water and nutrients (Akbar et al., 1996; Zur, 1996). Few computer simulation models have the capability to analyze water flow and nutrient transport in multiple spatial dimensions, with the exception of HYDRUS- 2D (Simunek et al., 1999; Cote et al., 2003) and FUSSIM2 (Heinen, 2001).HYDRUS-2D (Simunek et al., 2006) has been used extensively for evaluating short term nitrogen fertigation strategies and the effects of soil hydraulic properties, soil layering, dripper discharge rates, irrigation frequency, and timing of nutrient applications on wetting patterns and solute distribution (Ajdary et al., 2007; Patel and Rajput, 2008; Hopmans and Bristow, 2002; Doltra and Muñoz, 2010). Gardenas etal. (2005) analyzed four different micro irrigation systems in combination with five different fertigation strategies for various soil types, clearly demonstrating the effects of root distri- bution and fertigation strategy on the uniformity of water and nutrients around drip lines and their effects on water drainage and associated nitrate leaching by using the HYDRUS-2D. The soil wetting and solute transport in trickle irrigation