Modeling Nitrogen and Phosphorus Transport in Vadose Zone using HYDRUS-1D

The application of large manure on the agricultural lands that derived from the Concentrated Animal Feeding Operations (CAFOs) could cause excess nutrients such as nitrogen and phosphorus transport to the groundwater through vadose (unsaturated) zone. The objective of this study is as follows: (1) to compare nitrogen and phosphorus transport in the vadose zone at different Land Management Units (LMUs) for three consecutive years (2004-06) using HY-DRUS-1D, and (2) to analyze the sensitivity of nitrogen and phosphorus transport in different soil types on the van Genuchten soil hydraulic parameters: saturated water content ( θ s ), residual water content ( θ r ), alpha and n parameters, saturated hydraulic conductivity ( Ks ), adsorption isoterm coefficients ( Kd and β ), and tortuosity parameter ( l ). After modeling using HYDRUS-1D, it could be concluded that the transport of nitrogen was faster during the wet year. The concentrations of nitrogen compounds were most sensitive to saturated water content ( θ s ). Phosphorus was most sensitive to adsorption isoterm coefficient ( Kd ).


I. INTRODUCTION
The application of large manure on the agricultural lands that derived from the Concentrated Animal Feeding Operations (CAFOs) could cause excess nutrients such as nitrogen and phosphorus transport to the groundwater through vadose (unsaturated) zone. Due to nitrate pollution that contaminates the groundwater has become a particular concern. Therefore, it is important to know how nitrogen and phosphorus transport in the vadose zone.
Many studies have focused on phosphorus transport in the surface runoff and only a few have shown phosphorus transport that could occur in the vadose zone (Algoazany et al., 2007). Phosphorus transport through vadose zone is small and often neglected due to its slow mobility in the soils (Baker et al., 1975;Sharpley et al.,1993;Sims et al., 1998;Heathwaite and Dils, 2000;Hansen et al., 2002). Phosphorus transport through vadose zone is not considered as an important source of phosphorus movement to the groundwater (Sims et al., 1998;Hansen et al., 2002).
The objective of this study is as follows: 1. to compare nitrogen and phosphorus transport in the vadose zone at different Land Management Units (LMUs) for three consecutive years (2004-06) using HYDRUS-1D. 2. to analyze the sensitivity of nitrogen and phosphorus transport in different soil types on the van Genutchten soil hydraulic parameters: saturated water content (θs), residual water content (θr), alpha and n parameters, saturated hydraulic conductivity (Ks), adsorption isoterm coefficients (Kd and β), and tortuosity parameter (l).

II. LITERATURE REVIEW
Factors that affect nitrogen and phosphorus transport in the vadose zone can be divided into two factor; external and internal factors. The external factors are: 1. N and P application rate 2. Time and method of application 3. Rainfall timing and intensity Culley et. al, 1983;Edwards and Daniel, 1993;Sharpley et. al, 1993Sharpley et. al, , 1994.

Basic approaches to modeling nitrogen and phosphorus transport in the vadose zone
HYDRUS-1D is a one dimensional model to simulate vertical flow and solute transport in variably saturated soils under both steady-state and transient conditions (Šimůnek et al., 2008). In the vadose zone, lateral water fluxes can be assumed to be negligible and vertical flow dominates. Vertical flow and solute transport are described based on Richards equation and advection -dispersion equation.

Model approaches for Nitrogen and Phosphorus
Source Strength -the concentration of solute; Advection -movement of the solute in the direction of groundwater flow in accordance with Darcy's Law; Dispersionspreading of solute due to microscopic scale variations in flow and to molecular diffusion; Adsorption -reduction of solute in the groundwater caused by sorption of the species on to the soil; Decay -reduction of solute with time due to biological degradation, chemical reaction or radioactive decay (Freeze, 1989).
The forms of N in the soil, known as Chain Reaction: Ammonia (NH4)->Nitrite (NO2) ->Nitrate (NO3). The forms of P in the soil: dissolved and particulate, inorganic and organic. Dissolved inorganic P is considered in the leaching progress. Particulate organic P is not considered due to their complexities.

III. METHODOLOGY 3.1 Study Area
The study area consisted of two counties in Texas. Each county had one big dairy, approximately 100.000 cows, known as CAFO (Concentrated Animal Feeding Operations).  Figure 1 and 2 for Deaf Smith County and Erath County.

Soil Layers
Soil layers in both counties as described in Figure 3. Each soil layer had 200 cm in depth. The soil layers in Deaf Smith County, consisted of clay loam, clay, and clay loam, whereas in Erath Smith County, consisted of sandy loam, clay, clay loam, and sandy loam.  Figure 4. showed the inputs to model nitrogen and phosphorus using HYDRUS-1D. There were soil layers, water flow, and solute. The output was the profile information.

IV. RESULTS AND DISCUSSIONS 4.1 Nitrogen Transport
Nitrogen concentrations for the dry year were less compared to concentrations for wet and normal years.
These results were observed for both Deaf Smith and Erath counties. During the wet year, nitrogen transport in soil was found to be much faster. The liquid manure (wastewater) had given much more nitrate leaching than the dry manure. Nitrogen transport for dry and wet years was shown in Table 1 and nitrogen transport that derived from dry manure and wastewater described in Table 2.  Table 1. Nitrogen Transport in dry and wet years Table 2. Nitrogen Transport derived from dry manure and wastewater Table 3. Phosphorus Transport in dry and wet years

Phosphorus Transport
The leaching process of dissolved inorganic phosphorus concentrations for the dry year were less compared to concentrations for wet and normal years. These results were observed for both Deaf Smith County and Erath County (Table 3).

Sensitivity Analysis
The concentration of nitrogen compounds and was most sensitive to saturated water content (θs). By increasing 20%, the transport of nitrogen compounds was slower due to saturated condition (air entrapment). By decreasing 20%, the transport of nitrogen compounds was faster. Phosphorus was most sensitive to adsorption isoterm coefficient (Kd).

V. CONCLUSIONS
The conclusion of this study is as follows: 1. The transport of nitrogen was faster during the wet year. 2. The concentrations of nitrogen compounds were most sensitive to saturated water content (θs). 3. Phosphorus was most sensitive to adsorption isoterm coefficient (Kd).