How hydrology determines seasonal and interannual variations in water table depth, surface energy exchange, and water stress in a tropical peatland: Modeling versus measurements

Abstract

Soil carbon stocks in tropical peatlands have declined recently from water table depth (WTD) drawdown caused by increased frequency and intensity of climate extremes like El Niño and by artificial drainage. Restoration of these carbon stocks under these climatic and anthropogenic disturbances requires improved predictive capacity for hydrological feedbacks to ecological processes. Process-based modeling of tropical peatland ecohydrology could provide us with such capacity, but such modeling has thus far been limited. We aimed at using basic processes for water and O 2 transport and their effects on ecosystem water, carbon, and nitrogen cycling to model seasonal and interannual variations of WTD and surface energy exchange. We tested these processes in a process-based model ecosys in a drained tropical Indonesian peatland from an El Niño year 2002 to a wetter year 2005. WTD was modeled from hydraulically driven water transfers controlled vertically by precipitation versus evapotranspiration (ET) and laterally by discharge versus recharge to or from an external reference WTD. These transfers caused WTD drawdown and soil drying to be modeled during dry seasons, which reduced ET and increased Bowen ratio by lowering stomatal conductance. More pronounced dry seasons in drier years 2002-2004 versus wetter year 2005 caused deeper WTD, more intense peat drying, and greater plant water stress. These modeled trends were well corroborated by site measurements as apparent in regression statistics of modeled versus observed WTD (R 2 > 0.8), latent heat (R 2 > 0.8), and sensible heat (R 2 > 0.7) fluxes. Insights gained from this modeling would aid in predicting the fate of tropical peatlands under future drier climates. Key Points: • Ecosys modeled tropical peatland WTD by a vertical-lateral hydraulic scheme • Deeper WTD caused peat drying and reduced stomatal conductance in dry seasons • Stomatal limitation to transpiration was stronger in drier dry seasons Correspondence to: M. Mezbahuddin, mezbahud@ualberta.ca; symon.mezbahuddin@gov.ab.ca Citation: Mezbahuddin, M., R. F. Grant, and T. Hirano (2015), How hydrology determines seasonal and interannual variations in water table depth, surface energy exchange, and water stress in a tropical peatland: Modeling versus measurements, J. Geophys. Res.Given the effects of WTD drawdown from dry weather and artificial drainage on C storage, predictions of further drier weather by three global circulation models (GCMs) over tropical Southeast Asian peatlands by

Modeling hydrological controls on variations in peat water content, water table depth, and surface energy exchange of a boreal western Canadian fen peatland

“…Ecosys is a process‐based terrestrial ecosystem model that successfully simulated 3‐D water, energy, carbon, and nutrient (N, P) cycles across a variety of peatlands [ Dimitrov et al ., , ; Grant et al ., ; Mezbahuddin et al ., , ]. Ecosys algorithms that govern simulations of soil moisture retention, WTD, and surface energy exchange which are related to our hypotheses are described below.…”

“…Most of the current peatland models that do not simulate a prognostic WT use site‐measured WTD as inputs in these numerical solutions [e.g., Frolking et al ., ; Zhang et al ., ; Kurbatova et al ., ; St‐Hilaire et al ., ; Barr et al ., ]. A more complex process‐based model ecosys simulates a prognostic WT that affects soil moisture retention through a log‐transformed Campbell model (defined as a modified Campbell model or MCM hereafter), which enabled the model to simulate peat moisture retention reasonably well in a northern boreal bog [ Dimitrov et al ., ] and a tropical bog [ Mezbahuddin et al ., ]. The Campbell equation or its modification(s) (e.g., MCM in ecosys ) usually results in a hyperbolic relationship ( J shape) between soil moisture content ( θ ) and ψ m while simulating soil moisture desorption with declining ψ m (Figure ).…”

“…So to improve our predictive capacity of variable WTD feedback to ET across peatlands we need a more universal solution of peatland ET while equilibrating vegetation‐atmosphere moisture exchange with vegetation water uptake in a soil‐plant‐atmosphere hydraulic scheme. Instead of using site specifically parameterized scalar functions, this hydraulic scheme can equilibrate atmospheric ET demand from surface and canopy energy balances with moisture supply by vegetation as mediated by (1) rooting profiles resulting from root‐WTD interactions and (2) a series of water potentials (e.g., soil, root, and canopy water potentials) and hydraulic resistances (soil, root, canopy surface, and/or stomatal resistances) [ Dimitrov et al ., ; Mezbahuddin et al ., ].…”

“…Numerical solution of ψ m as a function of θ in a log‐transformed Campbell model (MCM; Figure ) enabled ecosys to successfully simulate near‐surface peat θ in a boreal bog [ Dimitrov et al ., ] and a tropical bog [ Mezbahuddin et al ., ] peatland. Those peats had low near‐saturation moisture‐holding capacity and hence exhibited rapid pore drainage immediately below saturation, thereby matching the J ‐shaped moisture retention curve in MCM when simulating decreasing θ with declining ψ m (Figure ).…”

Coupled eco-hydrology and biogeochemistry algorithms enable the simulation of water table depth effects on boreal peatland net CO<sub>2</sub> exchange

“…SD1-SD31). More detail about how peatland WTD, vertical and lateral soil water flow and soil moisture retention are modeled in ecosys can be found in Dimitrov et al (2010b) and Mezbahuddin et al (2015Mezbahuddin et al ( , 2016 (Flanagan and Syed, 2011); TN: total nitrogen (Flanagan and Syed, 2011); TP: total phosphorus (Flanagan and Syed, 2011); CEC: cation exchange capacity (Rippy and Nelson, 2007) yields, which drive microbial growth and substrate decomposition and uptake ( Fig. 1) (Eqs.…”