Phosphorus Modeling in Tile Drained Agricultural Systems Using APEX

Abstract

Phosphorus (P) losses through tile drained systems in agricultural landscapes may be causing persistent eutrophication problems observed in surface water. The purpose of this paper is to evaluate the state of the science in the Agricultural Policy/Environmental extender (APEX) model related to surface and tile P transport. This was accomplished using data from a monitored corn-soybean rotation field in the St. Joseph River watershed, IN. The estimation of soluble phosphorus (SP) in surface runoff and tile flow in APEX includes a user defined linear (based on GLEAMS) and nonlinear (Langmuir) sorption option. The results suggest that the inclusion of the Langmuir isotherm improved (18%) SP sorption estimates in surface runoff during the corn year only when P inputs were added, whereas the linear method was more appropriate during the soybean year when no fertilizers were applied. Similarly, SP estimates in tile flow were improved (30%) when using the Langmuir option during the corn year, though the overall model performance predicting this variable were very poor. Modeling improvements of P partitioning processes in APEX can help predict more realistic outputs. Yet to achieve this in tile flow, water percolation processes need to be improved to reflect preferential flow conditions often found in long-term no-till fields and in soils with high clay content. Greater accuracy in the estimation of the effect of artificial drainage systems, common in the US Midwest, should result in the improved evaluation of agricultural conservation practices in order to examine strategies that could reduce P losses for water quality purposes.

Modified APEX model for Simulating Macropore Phosphorus Contributions to Tile Drains

“…Of particular interest is the labile pool, which is defined as the soil mineral P currently available for plant use and leaching. Recently, APEX included use of the Langmuir adsorption isotherm, which could simulate the leaching of mineral P from the labile pool to the dissolved phase (Francesconi et al, 2016). Partitioning between DRP in soil and in liquid solution using the Langmuir isotherm was simulated as follows:…”

“…where C s was the labile DRP concentration in soil (mg P kg −1 ), C L was the DRP concentration in solution (mg P L −1 ), K was the Langmuir adsorption constant (L mg −1 ) at equilibrium and was parameterized (see Table 1) based on communications with the model developer and previously published best-fit values for tiledrained Midwestern systems (Francesconi et al, 2016), and S max was the maximum P sorption capacity (mg P kg −1 ) and was estimated as a function of clay content of the top soil layer for macropore tile P concentrations (Williams et al, 2012;Francesconi et al, 2016). The previously developed model assumed a single value for the Langmuir adsorption constant (K) for all soil layers given the potential for overparameterization of spatially discretized models.…”

“…where Q Matrix-P was the matrix DRP load to tile drainage (kg P ha −1 ) and was simulated as a function of the DRP leached to the layer containing the drainage system (Francesconi et al, 2016), and Q Macro-P was the macropore DRP load to tile drainage (kg P ha −1 ) and was simulated in the same manner as DRP loads for surface DRP loads with the Langmuir isotherm (Francesconi et al, 2016). The total amount of P present in a soil layer left for partitioning between the soil solid phase and soil solution in subsequent time steps was corrected for the amount of P lost to the next layer in the previous time step.…”

“…While APEX has been shown to adequately simulate field-scale hydrology and nutrient transport for surface runoff in tile-drained landscapes, some limitations persist in tile drainage simulations (Francesconi et al, 2014;Ford et al, 2015;Radcliffe et al, 2015). Recent studies have highlighted the inability of existing tile routines within APEX to simulate DRP loadings at the edge of field, which has been linked to the absence of macropore flow (Ford et al, 2015;Francesconi et al, 2016).…”

Evaluation of Phosphorus Site Assessment Tools: Lessons from the USA

“…The Annual P Loss Estimator is an empirically based, annual time-step model that requires minimal data and expertise to run but does not predict runoff or erosion, and thus these variables must be generated with additional models and entered as inputs into APLE. In each region, APEX0806 version was used, and thus recent advancements in P routines for APEX described in Francesconi et al (2016) were not used.…”