O. van Dam - WIT Press · 2014-05-15 · O. van Dam Dept. of Physical Geography, ... Development &...

Modelling solar radiation, evapotranspiration

and soil water dynamics in tropical rainforest

logging gaps in Guyana

O. van DamDept. of Physical Geography, Utrecht University, the NetherlandsTropenbos-Guyana Programme, Georgetown, Guyana

Abstract

Selective logging in the tropical rain forest of Guyana creates gaps in the canopy.The PCRaster model FORGAP was made to calculate the radiation energy,evapotranspiration and soil water dynamics of logging gaps. Model input datawas gathered in experimental gaps that differ in size. Calibration was done withmeasured soil moisture in the experimental gaps. The model proves a good toolin identifying areas within a gap that receive more light, show larger evapo-transpiration rates or experience more moisture stress. The model can be used inscenario studies of the effects of gap size, shape and orientation to the sun on themicroclimatic conditions and water availability that regulates forest regeneration.

1 Introduction

One of the main natural resources of Guyana is timber. Selective logging of thecommercial tree species creates openings or gaps in the canopy. Gaps are anatural feature of the forest caused by the falling of dead trees. Logging gaps areusually larger, because more than one tree is felled within a certain area andhealthy trees with mature crowns are felled. Furthermore, there are a largernumber of gaps per area in a logged forest than in an undisturbed forest. Theselogging activities change the microclimatic conditions and the water balance ofthe soil within the forest and within gaps [1]. Radiation, evapotranspiration andwater availability in tropical forest gaps are some of the discriminating factorsthat regulate the regeneration of the forest and the competition between treespecies [2]. There is a lack of knowledge on the effects of different sized gaps ormultiple gaps on the microclimate and soil water dynamics of a tropical rain

Development & Application of Computer Techniques to Environmental Studies VII, C.A. Brebbia, P. Zannetti & G.Ibarra-Berastegi (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-819-8

290 Computer Techniques in Environmental Studies

forest. The large variation between gaps of different size, shape and orientationto the sun and the within gap spatial variability can only be studied with a model.

The objective of this study is to identify the effect of gap size on themicroclimate and water availability. In 1996 the Tropenbos-Guyana Programmestarted the Pibiri Gap Experiment [3]. The experiment consists of the creation of25 experimental gaps in a range of 50 to 3200 nf. Measurements were made ofmicroclimatic conditions, soil moisture levels and nutrient availability. TheFORest GAP model FORGAP was made to study the effects of different gaps onthe radiation, evapotranspiration and soil water dynamics. This paper describesthe FORGAP model and the model performance.

2 The FORGAP model

FORGAP is written in the dynamic script modelling language of the PCRastersoftware [4]. PCRaster is a Geographical Information System that consists of aset of tools for storing, manipulating, analysing and retrieving geographic data. Itis raster-based and the software includes cartographic, dynamic and geostatisticalmodelling using a user-friendly modelling language. PCRaster is DOS based,using a simple ASCII editor. FORGAP consists of three modules: a radiation, anevapotranspiration and a soil water module. These are discussed separately.

2.1 The radiation module

The radiation module calculates the potential radiation on the vegetation, thepotential radiation on the saplings in the gap and in the area surrounding the gapand the potential radiation on the soil. The gap and the forest are modelled as adigital elevation model of the average height of the vegetation surrounding thegap and the height of the saplings in the gap. The radiation module calculates foreach hour the exact position of the sun in relation to the position of the studyarea with standard solar geometry [overview in 5]. This position determines thetotal radiation flux through a hypothetical plane perpendicular to the incomingsolar beams. The maximum solar radiation is corrected for the angle of incidentof the solar beams on the surface. In this study it represents the potentialradiation on the vegetation R g. For every hour of the day the model calculateswhich part of the gap is in the shade of the surrounding vegetation. The shadedpart of the gap only receives diffuse sky radiation and radiation that falls throughthe surrounding vegetation. A vegetated surface is unlike other surfacestranslucent. Solar beams penetrate through the openings between the leaves ofthe vegetation. A radiation extinction function is used to calculate the amount ofradiation that can penetrate through the vegetation. This extinction functiondepends on the leaf area index LAI (cm leaf area per cnf ground area) of thevegetation. Figure 1 shows a cross section of a hypothetical gap with radiationon the vegetation and inside the vegetation. The gap area is normally defined bythe perpendicular projection on the forest floor of the perimeter of the crownsthat surround the gap. However, the area on the forest floor that is influenced by


Computer Techniques in Environmental Studies 291

the presence of a gap is larger than this perimeter. Solar beams that fall into thegap can penetrate in the gap edge area, thereby increasing the actual gap area andthus the area where tree saplings can regeneration.

Forest Gap

Figure 1: Schematic representation of radiation in a forest gap.The abbreviations are explained in the text.

Radiation that falls on the forest soil arrives from two sources:• RLAI'. radiation penetrating through the small openings of the canopy treesleaves. The radiation is a function of the amount of radiation that falls on top ofthe canopy R^g (W.m~ ) and the Leaf Area Index LAI (nr.m ).• Rejge' radiation in the forest gap and gap edge. These solar beams can penetrateinto the forest and are being extinguished by the edge vegetation (stems, smalltrees and saplings) surrounding the gap. Radiation on the soil R.. (W/nT) underthe trees is a function of Rgdge, RLAI, an LAI extinction factor k (0.6 [6]), a gapedge radiation extinction constant c (0.18), the solar altitude a (deg) and thedistance from the gap edge D (m). Radiation on the saplings in the gap and in theforest undergrowth is calculated similar as /?,„//, but with the LAI above thesaplings instead of the total LAI. Hemispherical photographs were analysed withWinphot [5] to determine the constant c. These photographs were taken along atransect from the gap edge into the dense forest and the total radiation per hourfor 6 days in a year was calculated. Regression analysis was used to determine c.

2.2 The evapotranspiration module

Rain enters the forest system at the top of the vegetation. Rain is eitherintercepted by the vegetation or falls directly on the soil litter. Most of theintercepted water drips through to the soil litter (e.g. throughfall = drip + directthroughfall). Only a small proportion is left behind on the leaves or is drained tothe soil via the stems of the trees (e.g. stem flow). The water that remains behindwill evaporate. Water that was left behind on the soil litter will also evaporate.Transpiration by the vegetation and direct soil evaporation are only possiblewhen it is not raining. The evapotranspiration module calculates the netradiation, evapotranspiration fluxes (mm) and the net precipitation. The netradiation is computed by calculating the long wave radiation from microclimatic



data and correcting the potential radiation of the radiation module (which equalsthe short wave radiation) for cloud cover and albedo. The net radiation is used tocompute the potential transpiration and soil evaporation. The net precipitationplus the stem flow is the amount of water that enters the soil and that forms theinput for the soil water module. Temperature, relative humidity, air pressure,wind, cloudiness and rainfall are supplied as input for the model. The pathwaysof rainfall are given in figure 2.

loT

Pnet

Figure 2: Schematic representation of the evapotranspiration module.The abbreviations are explained in the text.

Canopy openness co (-) and litter openness lo (-) determine the amount ofrainfall and throughfall that is not intercepted by either the canopy leaves or soillitter. The storage capacity C^x (mm) or L ax (mm) determine the total amountof water that can be stored on respectively the canopy leaves or soil litter.Evaporation of intercepted water by the canopy Ei (mm), leaf litter El (mm) orpotential soil evaporation Es (mm) are calculated with Penman open waterevaporation. Potential transpiration Et (mm.h"') is calculated with Penman-Monteith [7]. Ei and Et are calculated with R^ and El and Es are calculated withRWJI. Canopy openness and LAI varies with tree height and tree density and wascomputed from hemispherical photographs. A literature value for Cmax wasused [0.89 in 8]. Litter openness varies with distance from the gap edge and wasmeasured in the field. Rainfall simulations were done to establish a relationshipbetween L ax and litter mass. Litter mass (g.m~ ) was measured in the forest andin the gap.

2.3 The soil water module

Water availability and water stress is calculated in the soil water module. Thesoil in the model consists of three layers up to the rooting depth of the vegetation(120 cm). Water percolating below this rooting zone is assumed lost from theforest ecosystem. The module is based on SWATRE [9], which calculates one-dimensional unsaturated flow. The module computes actual transpiration and soilevaporation related to the soil water suction and it is the minimum of the soilflux and the evaporative demand of the air (calculated in the previous module).



The model uses Darcy's law of one-dimensional stationary flow Q = -k(h) [8/z /5z - 1] (cm.h~*) in which &(h) is the hydraulic conductivity (cm.h"'), h pressurehead (cm) and z the gravitational head (cm). The pressure head h is a function ofthe soil moisture 0 and A: is a function of h according to the non-linear equationsof Van Genuchten and Mualem [10]. A flow chart of the WATBAL module isshown in figure 3.

1 iTemporarily surface sto" k ^i QDL, k..hu;

DL-, k,,, h^

D^ k^, KL,

Q:

Q,

Q,

iEpon !Es Et,,;V !

Et,: Et,,

Figure 3: Flow chart of the soil water balance module.See text for explanation of the abbreviations.

The amount of net precipitation P^, (mm) that enters the topsoil depends onthe maximum amount that can be stored in the top layer. The maximum storageis determined by the saturated soil moisture content 65 (-) and the amount that isalready present in the layer. If P^t (mm) is in excess of what can be stored in thetop layer, the excess water is stored temporarily on the surface and drains into thesoil in the next time step or evaporates E^n (cm). The amount of rain water thatenters the soil Qi (cm.h" ) is calculated with the hydraulic head and conductivityof the atmosphere (/z , rm) and hydraulic head and conductivity of the firstnode (hi, kj) with a z of V2 Dj (cm). The flux to the second layer gj is calculatedwith hydraulic potentials A/(6,) and 62(82) and subsequent hydraulicconductivities &/(/%/) and #2( 2) and az of (D; + D?)/2. The flux to the third layergj is likewise computed. The flux below the rooting zone Qj is computed withthe h and k of the third layer of the previous time step. After the calculations ofall fluxes the water extraction of each layer by evapotranspiration is determined.Actual soil evaporation Es (mm) is only present in the top layer. The actualtranspiration of each layer Eta (mm) depends on the evaporative demand of theair, given by the potential evapotranspiration Etp^ (mm), and the pressure headof that soil layer. The pressure head is needed in a transpiration reductionfunction that limits transpiration at dry or wet soil moisture conditions [9].Finally, the new soil moisture conditions for the following time step arecalculated.

3 Calibration

Field measurements were made between 1996 and 1999 in the experimentalgaps. Microclimate data that is used as input for the model was monitored in a



large gap (3200 nf). Vegetation and soil parameters were measured at study sitesalong gap-centre to forest transects. Volumetric soil moisture was measuredweekly with a Trime FM-2 TDR tube sensor in PVC tubes of one meter length.Soil moisture measurements were used to calibrate the soil water module.Calibration of the model was facilitated with the computer program PEST.

Model parameter optimalisation was performed with the computer programPEST [11]. PEST is a model-independent computer program for ParameterESTimation. For linear models (i.e. models for which observations are calculatedfrom parameters through a matrix equation with constant parameter coefficients),optimisation can be achieved in one step. However for non-linear problems(most models fall into this category), parameter estimation is an iterative process.PEST uses the Gauss-Marquardt-Levenberg algorithm [11] to solve the non-linear weighted least squares parameter estimation.

A large number of FORGAP parameters could be optimised with PEST. Asensitivity analysis of FORGAP was done for several vegetation and soilparameters. Parameters were selected that showed the largest influence in soilmoisture and that were backed with only limited field data. Vegetation heightwas the only above ground parameter that had any significant influence on soilmoisture conditions. Soil parameters that were selected are: saturated hydraulicconductivity, saturated volumetric soil moisture, Mualem's n and a. The user ofPEST provides information on the upper and lower boundaries of the parametersthat are being optimised. The boundaries that were supplied originate fromvalues measured in the field by the author and by Jetten [8].

4 Output examples

4.1 Radiation

The year sum of potential radiation on the vegetation, saplings and soil in a largegap (3200 m^) is shown in figure 4. Total radiation on the vegetation isuppermost on the trees surrounding the gap. Total radiation increases from thegap edge to the gap centre.

i

8295641845242638752

Figure 4: Year sum of potential radiation (MJ.m ) on A) the vegetation, B) thesaplings and C) the soil.



Total radiation on the vegetation in the gap centre is 85% of total maximumradiation. Total soil radiation in the gap centre is 80% of total radiation on thesaplings. Total radiation on the saplings and on the soil in the forest undergrowthis about 10% of the total radiation above the forest. Total sapling and soilradiation decreases with increasing distance from the gap edge. Saplings in a partof a large gap that is extending into the forest, like at the top of the gap in figure4, only receive 45% of the amount of radiation of the saplings in the centre ofthat gap.

4.2 Evapotranspiration

Figure 5 shows the evapotranspiration fluxes as percentage of annual rainfall ofundisturbed forest and in a large 3200 nf gap. Interception, throughfall and stemflow of the undisturbed forest has comparable values as reported by Jetten [8].The El Nino event in the latter half of 1997 contributes to the high potentialevapotranspiration, which normally is about 50-60 % of total rainfall.Interception and transpiration is less in the gap than in the forest. Throughfall,soil evaporation and percolation are larger in the gap than in the forest.

1008060 -4020 -0 _n

agap

Ei Th St El Eta Esa Loss Ep

Figure 5: Interception evaporation (Ei), throughfall (Th), stem flow (St), litterevaporation (El), actual transpiration (Eta) and soil evaporation (Esa),percolation below the rooting zone (Loss) and potential evapotrans-piration (Ep) in a forest and a gap centre site expressed as % of annualprecipitation.

4.3 Soil water dynamics

The effect of gap size on the number of time steps (hours) that the soil moisturesuction in the topsoil is lower than -1000 cm is shown in figure 6. Tropical rainforest gaps are usually wetter than the surrounding forest due to a reducedtranspiration in the gap [9]. However, very large gaps have a larger soilevaporation that is causing dryer conditions in the topsoil. As can be seen infigure 6, the small gaps experience wetter conditions than the surrounding forest.However, in the large gaps dryer conditions occur more often in the gap than theforest. Dryer conditions also prevail in the gap edge compared to the forest. Theactual area that is under influence of the gap is larger than the perpendicularprojection of the hole in the canopy.



Figure 6: Effect of gap size on the occurrence (hours per year) of soil watersuctions lower than -1000 cm.

5 Discussion and conclusions

5.1 Calibration

The parameter estimation of PEST yielded soil hydrological parameter valuesagainst their upper or lower boundary. This implies that most likely a betteroptimisation can be achieved with hydrological parameter values outside theseboundaries. However, the integrity of the model parameters with real fieldparameter values would be lost.

Does this mean that the model does not perform well? There are severalpossible explanations for the lack of correlation between measured andcalculated soil moisture. Jetten [8] studied the spatial variability of several soilhydrological parameters, including the soil moisture of the topsoil, incomparable soil types as present in the study area. He reported a high short-range(2-20 m) variation for most soil hydrological parameters. Detailed information ofsoil hydrological parameters would be required for every grid cell. Thisinformation is not available and the variation in the output is thereforeimpossible to model. Another problem is associated with the TDR soil moisturemeasurements. TDR soil moisture measurements in PVC tubes in wet tropicalareas are sensitive for a) the position of the two metal strips inside the tube, b) airpockets between the tube and the soil matrix and c) moist conditions inside thetube or on the metal strips. Calibration of all tubes at all depths (up to 1 m) isnecessary, but was not possible due to technical malfunctions. The prolongedwet season in 1999 prohibited the measurement of a wide range of soil moistureconditions needed for a good calibration. The soil moisture measurements withthe tube TDR probe have to be treated with caution.

5.2 Model performance

The potential radiation on the vegetation is calculated with solar (geometry)equations that are widely accepted. The derivation of the soil radiation is basedon own observations and analysis. Radiation on the soil and on the saplings inthe gap and in the gap edge gives insight into the spatial variation within andbetween gaps. Sapling radiation can be a discriminating site characteristic thatexplains sapling demography differences. The evapotranspiration functions are



used in many forest models [9,12]. The magnitude of the evapotranspirationfluxes of FORGAP and for example SOAP [8,12], which was developed formodelling the water balance of a tropical rain forest, are comparable.Unfortunately there were no field measurements available to calibrate theevapotranspiration module. The modelling of unsaturated flow is an iterativeprocess. A change in soil moisture leads to a change in water potential andconductivity that results in changes of the soil moisture. The iteration mustcontinue until a satisfactory balance in a soil layer is established. This iterativeprocess cannot be modelled with PCRaster. The basic unsaturated flow dynamicsof the model are sound. It was not possible to match the soil moisture modeloutput with the large spatial variability of the soil moisture measurements.However, FORGAP is a useful tool in providing insight into the coheringprocesses of microclimate and water cycling in tropical rainforest gaps. Themodel can be used to explain differences in water availability between andwithin gaps and provide insight into the processes that gave reason for thesedifferences.

5.3 Future developments

The effects of tropical rain forest gaps in Guyana will be studied in scenariostudies with hypothetical gaps. These scenario studies will focus on the effect ofgap size, the effect of gap orientation to the sun, the effect of gap shape and theeffect of multiple gaps. A multiple gaps approach is the most realistic view, sincea lot of small gaps are usually made in real logging operations. These small gapstogether will act as a very large gap.

FORGAP can be improved with the inclusion of a plant growth module and arun-off module. Currently the saplings in the gap have a growth relation with netradiation that is only valid for the first 2 years after gap creation. A plant growthmodel can predict the growth of the saplings in the gaps and thereby make themodel useful for long-term predictions. The soil water module only calculatesvertical flow. The model can only be used in flat terrain. The expansion to lateralflow and surface run-off could make the model useful for other soil types inmore sloping terrain.

The radiation module and evapotranspiration module can be used separatelyto calculate radiation and evapotranspiration of entire regions. The model mustbe linked to a GIS with information on topography and land use as well as thenecessary model parameters per land unit. Detailed calculations related to forestgaps can be left out to improve the velocity of the model calculations. Thestructure of FORGAP can be adjusted to the environmental settings of anyterrain.

AcknowledgementsThe author wishes to thank the people at the Tropenbos-Guyana Programme fortheir co-operation and support with the fieldwork. Special thanks to dr. V. Jetten



for assistance with the model and dr. H. ter Steege for the useful comments on anearlier version of the manuscript.

References

[1] Ter Steege, H. et al. (19 authors) Ecology and logging in a tropical rainforest in Guyana. With recommendations for forest managers. TropenbosSeries 14. The Tropenbos Foundation, Wageningen, the Netherlands. 1996.

[2] Denslow, J.S. Gap partitioning among tropical rainforest trees. Biotrop. 12:47-55 (spec, ed.), 1980.

[3] Van Dam, O., Rose, S.A., Houter, N.C., Hammond, D.S., Rons, T.L., & terSteege, H. The Pibiri Gap Experiment. A study of the effects of gap size onmicroclimate, edaphic conditions, seedling survival and growth,ecophysiology and insect herb ivory. Site description, methodologies andexperimental set-up. Tropenbos-Guyana Interim Reports 99-1, Tropenbos-Guyana Programme, Georgetown, Guyana, 1999.

[4] PCRaster PCRaster manual Second edition. Utrecht University, dept. ofPhysical Geography, 1995.

[5] Ter Steege, H. Winphot. A Windows 3.1 programme to analyses vegetationindices, light and light quality from hemispherical photographs.Tropenbos-Guyana Reports 97-3. Tropenbos-Guyana Programme,Georgetown, Guyana. 1997.

[6] Whitmore, T.C., Brown, N.D., Swaine, M.D., Kennedy, D., Goodwin-Bailey, C.I., & Gong, W.-K. Use of Hemispherical photographs in forestecology: measurement of gap size and radiation totals in a Bornean tropicalrain forest. J. Trop. Ecol 9, pp. 131-151, 1993.

[7] Monteith, J.L. Evaporation and the environment. The state and movementof water in living organisms. Proc. of the XIX symp. of the Soc. For Exp.Biol., Swansea, Cambridge Uni. Press., pp. 205-234, 1965.

[8] Jetten, V.G. Modelling the effects of logging on the water balance of atropical rain forest. A study in Guyana. Thesis. Dept. Of PhysicalGeography, Utrecht University, Tropenbos Series 6, The TropenbosFoundation, Wageningen, the Netherlands. 1994

[9] Feddes, R.A., Kowalik, P.J. & Zaradny, H. Simulation of field water useand crop yield. Simulation Monographs. Centre for Agricultural Publishingand Documentation, Wageningen, 1978.

[10] Mualem, Y. A new model for predicting the hydrological conductivity ofunsaturated porous media. Water Res our. Res. 12, pp. 513-522, 1976.

[11] Watermark Computing PEST. Model-independent Parameter Estimation.Manual computer program, Watermark Computing, 1994.

[12] Jetten, V.G. SOAP. SOU Atmosphere Plant model. A one dimensionalwater balance model for a forest environment (Theoretical framework &Manual) Dept. of Physical Geography, Utrecht University, the Netherlands.The Tropenbos-Guyana Programme, Georgetown, Guyana, 1994.


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