# HYDRUS-1D Simulation of Nitrogen Dynamics in Rainfed Sweet Corn Production
## 1 Introduction  
Solute transport in agricultural fields is dynamic in nature. Nitrogen (N) pollution has become a global environmental problem with serious implications on surface and ground waters [1]. The groundwater contamination due to agricultural activity depends on the amount of N applied and its effective use by crops [2]. N is an essential nutrient that highly affects crop yield [3]. Therefore, farmers apply N-fertil to high-yield crops. Corn also demands large amount of N for optimal yield. Due to a lack of management guidelines, most farmers apply fertiliser based on their experience and do not consider their environmental consequences [4]. N leaching out of the root zone due to excessive application of N-fertilisers is a potential cause of water resource pollution, which has been observed in many parts of the world [5, 6].  
In humid regions, low recovery of fertilizer by crop results in increased leaching of soil residual N to groundwater during off-season rainfall [7]. Rainfall triggers flow processes such as surface runoff, preferential flow, and the nitrate leaching [8; 9]. Excessive water application is also a major factor that controls N leaching. Therefore, along with fertiliser optimum use of water is also a prerequisite. In soil, N is present in different forms such as organic N, ammonia (NH\({}_{4}\)\({}^{+}\)-N) and nitrate (NO\({}_{3}\)\({}^{-}\)-N). Due to its mobility, NO\({}_{3}\)\({}^{-}\)-N contributes to groundwater pollution more than any other form [9]. In the past, many research studies have been carried out to find the effect of reduced fertiliser rate on the crop. However, the reduction in the use of fertiliser may reduce the yield as well due to which the farmers are less willing to adopt this technique. Reducing the water application could be an option, particularly in tropical regions to reduce fertiliser leaching. Therefore we considered rainfed conditions and ignored scheduled irrigation. The current study focused to evaluate the effect of rainfall on nutrient distribution in the tropical region based on intensive fieldwork.  
The simulation models have been very effective in describing transport processes and the extent to which management practices affect crop yield and the environment. However, the significance of their use is multiplied when the prediction is based on local soil and climate conditions [10]. Among different available models, HYDRUS-1D [11] has been widely used for water flow and solute transport simulations, and to analyze flow and transport processes in agricultural fields. HYDRUS-1D has the flexibility of accommodating different boundary conditions as compared to other models. The model can consider the root uptake of water and nutrient simultaneously. The model is capable of simulating soil water and solute dynamics under different management practices [12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23].  
The heavy rainfall events in the tropical climate cause a significant increase in solute loss. In Malaysia, the amount of rainfall may potentially fulfil the crop water requirements to ensure nitrogen availability to plant. However, the uncertainty in the duration and frequency of rainfall can affect nitrogen distribution and crop uptake. Therefore, the study was carried out to quantify nitrogen losses through surface runoff and subsurface leaching from the sweet corn field. The study objectives were: (1) to evaluate the rainfall impact on nitrogen distribution and losses under tropical rain-fed conditions in presence of shallow water table, and (2) to calibrate and validate the HYDRUS-1D model for nitrogen transport in the humid tropics.