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<ul><li><p>JOURNAL OF TIlE AMERICAN WATER RESOURCES ASSOCIATIONVOL. 34, NO.4 AMERICAN WATER RESOURCES ASSOCIATION AUGUST 1998</p><p>MODELING THE FOREST HYDROLOGY OFWETLAND-UPLAND ECOSYSTEMS IN FLORIDA'</p><p>Ge Sun, Hans Riekerk, and Nicholas B. Comerford2</p><p>ABSTRACT: Few hydrological models are applicable to pine flat-woods which are a mosaic of pine plantations and cypress swamps.Unique features of this system include ephemeral sheet flow, shal-low dynamic ground water table, high rainfall and evapotranspira-tion, and high infiltration rates. A FLATWOODS model has beendeveloped specifically for the cypress wetland-pine upland land-scape by integrating a 2-D ground water model, a Variable-Source-Area (VAS)-based surface flow model, an evapotranspiration (ET)model, and an unsaturated water flow model. The FLATWOODSmodel utilizes a distributed approach by dividing the entire simula-tion domain into regular cells. It has the capability to continuouslysimulate the daily values of ground water table depth, ET, and soilmoisture content distributions in a watershed. The model has beencalibrated and validated with a 15-year runoff and a four-yearground water table data set from two different pine flatwoodsresearch watersheds in northern Florida. This model may be usedfor predicting hydrologic impacts of different forest managementpractices in the coastal regions.(KEY TERMS: forest hydrology; Florida; ground water hydrology;pine flatwoods; modeling; wetlands.)</p><p>INTRODUCTION</p><p>The southeastern United States has always been acenter for soft wood timber production due to its idealclimatic and soil water conditions for tree growth(Sabine, 1994). More than one million acres in thecoastal area are classified as pine flatwoods (Cubbageand Flather, 1993). Important values of this ecosys-tem include timber production, wildlife habitat, andgroundwater recharge (Brandt and Ewel, 1989). Ashuman population and wood demands continue to risein the southeastern United States, forestry activitieson pine flatwoods have become increasingly intensi-fied (Riekerk and Korhnak, 1984). Environmental</p><p>concerns about forest management practices in thecoastal areas include impacts on water quality, wet-land hydrologic functions, resultant influences onwildlife habitat, and long-term cumulative impacts onsoil productivity. Although the effects of forest man-agement on upland watershed hydrology have beenwell studied in the past century (Bosch and Hewlett,1982; Swank and Crossley, 1988), little information isavailable for the low-land and forested wetland land-scape (Riekerk, 1989; Shepard et al., 1993).</p><p>Hydrologic computer simulation models are becom-ing essential tools for scientists as well as land man-agers in the decision making processes (Lovejoy etal., 1997). Although the building procedures and com-ponents of existing hydrologic simulation models aresimilar, one model may be very different from anotherin its capability and applicability. For example, mostof the available hydrologic models developed for hillyregions are not applicable to Florida's coastal condi-tions (Heatwole et al., 1987; Capece, 1994). Also, mod-els developed for agricultural watersheds often needsignificant modification before they can be applied toforests (Thomas, 1989; McCarthy et al., 1992). Severalefforts have been made in modeling flatwoods hydrol-ogy. Heatwole et al. (1987) modified the CREAMSmodel (Knisel, 1980) into CREAMS-WT to better rep-resent the storage-based flatwoods hydrologic systemof agricultural watersheds in southern Florida. Basedon the agricultural drainage model DRAINMOD(Skaggs, 1984), a forest hydrology version DRAIN-LOB was developed to model water managementeffects on the hydrology of loblolly pine plantation inNorth Carolina (McCarthy et al., 1992). The DRAIN-MOD model was also modified to a new model, Field</p><p>1Paper No. 97120 of the Journal of the American Water Resources Association. Discussions are open until April 1, 1999.2Respectively, Research Associate, North Carolina State University/USDA Forest Service, 1509 Varsity Dr., Raleigh, North Carolina</p><p>27606; Associate Professor (retired), School of Forest Resources and Conservation, University of Florida, Gainesville, Florida 32611; and Pro-fessor, Department of Soil and Water Science, 2169 McCarty Hall, University of Florida, Gainesville, Florida 32611 (E-Mail/ OF THE AMERICAN WATER RESOURCES ASSOCIATION 827 JAWRA</p></li><li><p>Sun, Riekerk, and Comerford</p><p>Hydrologic And Nutrient Transport Model(FHANTM), to simulate the hydrology and phospho-rus movement on flatwoods fields (Tremwel andCampbell, 1992; Campbell et al., 1995). Limited suc-cess was found in directly applying an upland event-based forest hydrological model (VSAS2) ( Bernier,1982) to a pine flatwoods watershed (Guo, 1989). Itwas suggested that the flat topography and largecypress wetland storage significantly reduced storm-flow under most situations. Hydrologic cycles areincluded in most of the forest ecological models forslash pine plantations, but water movement processesare not physically and explicitly modeled (Golkin andEwel, 1985; Ewel and Gholz, 1991).</p><p>In recent years, there has been a tendency to devel-op more physically based distributed models (Beven,1989; Jensen and Mantoglou, 1992; Bathurst andO'Connell, 1992; Wigmosta, 1991) and couple hydro-logic processes with biological processes (Al-Soufi,1987; Band et al., 1993; Wigmosta et al., 1994). Thisdevelopment has been driven by the need for compre-hensive large-scale (e.g., basin, global) ecosystemstudies, in which the hydrology is one of the mostimportant components (Swank et al., 1994; Pierce etal., 1987), and has been accelerated by the increase ofcomputation power. The advance of Geographic Infor-mation Systems (GIS) technologies makes it possibleto develop large-scale databases and interpret com-plex model outputs (Maidment, 1993).</p><p>The Florida flatwoods landscape is a mosaic ofcypress wetlands and forest uplands; therefore thehydrology of flatwoods is inherently heterogenous andcomplex (Figure 1). Examples are: (1) the slight spa-tial changes in topographic elevation causing signifi-cant changes in the water regime, and (2) obstructivesoil layering because of the spodic and argillic hori-zons in the soil profile. The heterogeneous vegetationcover of wetlands and uplands and associated phenol-ogy may further complicate the interactions betweensurface water and ground water. Due to the complexgeologic formation of flatwoods, the preferential waterpathways under different management conditions ofthis system have not been well documented or under-stood. A new distributed flatwoods forest hydrologicmodel is needed to study the hydrologic processes ofwetland/upland systems and provide a tool to evalu-ate the hydrologic impact of forest harvesting, specifi-cally for this landscape. The South Florida WaterManagement District has proposed a conceptual inte-grated hydrologic model for Dade County in southernFlorida. This area is characterized by flat topography,sandy soils, a shallow ground water table, and welldeveloped canal systems (Yan and Smith, 1994).Unfortunately, the model has not been implementedfor the intended use in regional water supplyplanning. Based on MODFLOW (McDonald and</p><p>Harbaugh, 1984) and BROOK (Federer and Lash,1978), the COASTAL model was developed for model-ing large coastal basins (Sun, 1985). Distributedstructure and major hydrologic components of thismodel fit the pine flatwoods hydrologic system, butseveral algorithms have to be rewritten to modelforested landscape. Advantages and disadvantages ofthe COASTAL model have been discussed in detail inSun (1995).</p><p>Based on its basic structure, a new forest hydrolog-ical model, FLATWOODS, was developed and validat-ed for pine flatwoods to: (1) predict spatial andtemporal hydrologic effects of forest managementpractices; (2) account for hydrologic heterogeneity andcontinuity of wetland/upland ecosystems and environ-mental variables; and (3) provide a tool for forestwater management and hydrologic research. Thispaper describes model development procedures andpresents model calibration and validation results.</p><p>Structure</p><p>MODEL DEVELOPMENT</p><p>Recognizing the heterogeneity of the flatwoodslandscape, the model imposes a grid over the entirewetland-upland system to discretize the heteroge-neous watershed into different, but homogeneousrectangular cells (Figure 2). The physical properties ofeach cell are assumed to be uniform laterally for eachsoil layer, but non-uniform vertically in different soillayers. Each cell becomes a modeling unit that holdsmathematical equations describing the physical prop-erties. In practice, spatial data for forest lands arerarely available at high resolution with the exceptionof some readily available parameters such as surfaceelevation, vegetation and soil types (wetlands vs.uplands), etc.</p><p>Hydrologic Components and Governing Equations</p><p>The model consists of four major submodels to sim-ulate the full hydrologic cycle including: (1) evapo-transpiration, (2) unsaturated water flow, (3) groundwater flow, and (4) surface flow (Figures 2 and 3). Themodel simulates the hydrologic processes on a dailytime step. Computer codes were written in one pro-gram in FORTRAN language, which integrates allfour submodels. The flow chart describes the relation-ships among the submodels (Figure 3).</p><p>JAWRA 828 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION</p></li><li><p>Modeling the Forest Hydrology of Wetland-Upland Ecosystems in Florida</p><p>Figure 1. One Research Site on a Typical Pine Flatwoods LandscapeShowing the Spatial Relations of Wetlands and Uplands.</p><p>W+E</p><p>Evapotranspiration</p><p>Evapotranspiration (ET) is the most importantcomponent of pine flatwoods water balance (Ewel andSmith, 1992; Sun et al., 1995). Driving forces for thehydrologic system are climatic variables includingrainfall and air temperature (evapotranspiration).The daily rainfall and temperature data as modelinputs of climatic variables are available from actualfield recordings or the local weather station. The ETsubmodel has three components including a rainfallinterception component (In) by forest canopies, evapo-ration from soil/water surfaces, and transpirationthrough plant stomata. The rainfall interceptiondepends on daily rainfall, leaf area index (LAI) andavailable canopy interception storage or dryness ofthe forest canopy.</p><p>Rainfall interception is modeled using an empiricalequation:</p><p>I=a+bxP (1)where = rainfall interception (mm/day); P = grossdaily rainfall (mm/day); and a and b = coefficients fit-ted from throughfall measurements.</p><p>The maximum interception or canopy storagecapacity Cmax in mm was introduced to make theempirical model (Equation 2) more realistic. Canopystorage capacity was determined by assuming thatcanopy saturation was approximated by a 1.0 mmthick water film over the foliage surfaces:</p><p>C5=1.OxLAI (2)</p><p>JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 829 JAWRA</p><p>Gator National Forest</p><p> WellContour linePine UplandsClress Wetlands</p></li><li><p>Sun, Riekerk, and Comerford</p><p>0a)4</p><p>a)a)U)</p><p>C0</p><p>a)C)a)a)a).C a).</p><p>OEo</p><p>0</p><p>I</p><p>a)0E</p><p>CC0</p><p>0a)</p><p>0- a)</p><p>-</p><p>C</p><p>-</p><p>C</p><p>0-C(JD</p><p>Cl)</p><p>00</p><p>C)</p><p>0C</p><p>CI)C')C)</p><p>JAWRA 830 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION</p><p>0a)</p><p>0</p></li><li><p>Modeling the Forest Hydrology of Wetland-Upland Ecosystems in Florida</p><p>Then, available canopy storage for rainfall intercep-tion (ASIN) is calculated as:</p><p>ASIN = C8 + PET - SIN</p><p>where SIN = water stored on the forest canopy theday before the simulation date (mm); PET =potentialevapotranspiration (PET) is estimated by Hamon'smethod (Hamon, 1963; Federer and Lash, 1978).</p><p>A PET correction coefficient of 1.3 was found to beappropriate for the Florida climatic conditions as com-pared 1.0 used for Hubbard Brook, New Hampshireand 1.2 for Coweeta, North Carolina (Federer andLash, 1978).</p><p>ASIN = C8</p><p>I = a + b x PI, = ASIN</p><p>If PET&gt; SIN</p><p>If I &lt; ASIN</p><p>If I &gt; ASIN</p><p>I = 0 If ASIN = 0</p><p>Throughfall (Tn), T = P - I, represents rainfallthat passes through all vegetation layers and becomesavailable for infiltration into the ground. Variation ofthe leaf area index (LAI) of cypress wetlands andslash pine plantations as a function of Julian day (t)were derived from measurements (Liu, 1996). For theslash pine forest at the Gator National Forest site(GNF),</p><p>LAI(t)= 2. 25+0. 25xsin(--t+ L 1t365 (3)</p><p>For mature slash pine plantations, a maximum LAIand transpiration rate were assumed (Gholz et al.,1991):</p><p>MLAI(t)= 5.0+0.83xsinI--t+it' 365</p><p>(4)</p><p>For the cypress wetlands at the research site (Liu,1996):</p><p>LAI(t) = 1.2 If t &lt; 60</p><p>LAI(t) = 1.2 + (3.93 - 1.20)!(121 - 61) t = 1.2 + 0.0455 t</p><p>LAJ(t)= L5+4.4xsin(.Tht+1.97cJ</p><p>If 60&lt; t = 121</p><p>Figure 3. FLATWOODS Model Flow Chart Showingthe Relationships Between the Submodels.</p><p>Cypress forests at the sites of the present studywere assumed to be in the mature stage with themaximum leaf area index.</p><p>Evaporation of intercepted water on forest canopieshas first demand on PET. Residual potential evapo-transpiration (RET) is the difference between PETand intercepted water evaporation from plant sur-faces. Actual evaporation (AE) from soil!water sur-faces is dependent on atmospheric demand, soil waterconditions, and is affected by forest canopy shading.Actual transpiration (AT), which involves physical</p><p>JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 831 JAWRA</p></li><li><p>Sun, Riekerk, and Comerford</p><p>F1 = 1.0</p><p>CT</p><p>F = i_10i,f_0i')IOi,191,w)</p><p>and physiological processes, is the most difficult com-ponent to model. In the FLATWOODS model, ATextracted from each of the soil layers is a function ofpossible realized transpiration (PRT), soil water con-ditions, and root density. The concept of PRT isdefined as the maximum transpiration that a crop canhave for a certain atmospheric condition and LA!.PRT is a function of the residual potential evapotran-spiration (RET), and stage of plant development indi-cated by LAI and root density.</p><p>Solar energy is first consumed by evaporation ofthe rain water intercepted on the forest canopy beforereaching the ground. Residual potential evapotranspi-ration (RPET) has been defined as the differencebetween PET and SIN. Evaporation from the soil sur-face is also reduced by forest canopy shading(McCarthy et al., 1992). Field data suggest that evap-oration from a floating pan under the cypress wetlandcanopies of the study area was only about 30 percentof the standard pan evaporation (Liu, 1996).</p><p>PE = RPET x exp [-KE1 x LAI(t)] (5)where PE = potential evaporation (mm/day); RPET =residual potential evapotranspiration (mm/day) =PET - SIN; LAI(t) = leaf area index on Julian day t;and KE1 = soil evaporation reduction coefficient.</p><p>Actual evaporation (AE) is modeled as a function ofwater table depth (WDEP) in two phases:</p><p>Phase 1: Atnisphere DependentAE=PE IfWDEP =</p><p>_______Ife .</p><p>:eo(0 't) - .- 0</p><p>JAWRA 836 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION</p><p>(0</p><p>tN</p><p>t)</p><p>0</p><p>c1</p><p>N</p><p>(0</p><p>a</p><p>0</p><p>Cl</p><p>Cl</p><p>Cl)-C</p><p>0Cl</p><p>'2)</p><p>&gt;,U)</p><p>-Cl</p><p>-ClaNClC(0</p><p>&gt;&lt; aClCl</p><p>C00r..)</p><p>00 O N 0(0 u) P' C1 .- 0W3SIS PJO A</p></li><li><p>Modeling the Forest Hydrology of Wetland-Upland Ecosystems in Florida</p><p>specific...</p></li></ul>


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