Study of the changes of hydrological processes induced by afforestation in mediterranean mountainous abandoned fields

Phys. Chem. Earth, Vol. 20, No. 3-4, pp. 375-383, 1995, Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0079-1946(95)00051-8 0079-1946/95 $9.50 + 0.00

Pergamon

Study of the Changes of Hydrological Processes Induced by Afforestation in Mediterranean Mountainous Abandoned Fields P. Llorens, R. Poch, D. Rabada and F. GaHart

Institute of Earth Sciences' Jaume Almera', CSIC, P.O. Box 30102, 08080 Barcelona, Spain

ABSTRACT Ongoing experimental research is conducted to analyze the differences of water balance processes between grassed and forested patches in an area where the abandonment of agricultural activities facilitates the spontaneous overgrowth of pine forest. The ultimate aim of this work is to test and adapt the findings and models recently developed about forest interception in wetter climates for Mediterranean mountain environments, where water resources are crucial and threatened by environmental change. Provisional results demonstrate that the higher rainfall interception in forest patches dearly dominates the water balance, but common physically-based interception models seem to fail to account for the observed interception rates.

KEYWORDS Evapotranspiration, Rainfall interception, Forest hydrology, Water resources, Environmental change, Forestation, Mediterranean mountain.

INTRODUCTION The abandonment of marginal agricaltural fields since early this century has been one of the main geoecological modifications taking place in mountainous Mediterranean areas, it is a process still encouraged by current agricultural policies. This implies that in the near future more than 15.000 ha of low productivity agricultural lands will be abandoned in the EC countries (Miller, 1992), Following abandonment, the main geoecological adjustment that has or will be produced in these areas is either spontaneous or promoted overgrowing of forest land cover. Where forestation takes place in these Mediterranean mountainous areas there is considerable uncertainty about the impact of this land cover change on the water resources of the region. There is increasing agreement among authors that forest land cover results in higher water losses to the atmosphere than grassland (Bosch & Hewlett, 1982) and that this is the result of increased rainfall interception (Rutter, 1967). Nevertheless, most of the studies from which these conclusions are drawn were conducted in temperate humid areas, with weather regimes and rainfall intensifies very different from Mediterranean ones.

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Following from these considerations there are two main paths to follow: the first is to increase the knowledge of interception mechanisms in Mediterranean areas, and the second is to develop techniques to generalize the mechanisms involved in such processes for hydrological modelling purposes.

THE STUDY OF THE WATER BALANCE IN A MEDITERRANEAN AREA The Study Area The Cal Parisa basin (36 Ha), located in the headwaters of the Llobregat river (Eastern Pyrenees, Spain) at 1400 m a.m.s.l., was selected as representative of abandoned agricultural fields in Mediterranean mountainous areas (Llorens & Gallart, 1992). The climate is Mediterranean mountainous, with a mean annual precipitation of 850 mm and a mean annual temperature of about 9 °C. Since 1989, the hydrological response of the more man-modified sub-basin has been studied to analyze the role of land use changes in water and land conservation. The main geoecological implications of the earlier agricultural land use were marked modifications to the vegetation cover, topography and drainage net, with significant impact on geomorphical and hydrological behaviour (Llorens et al. 1992, Gallart et al. 1994). After the abandonment of agricultural fields, the forested area of the basin has increased from 5% to 25% in the last twenty years. Spontaneous reafforestation seems to be the main geoecological change that may modify the hydrological response of the basin (Llorens, in press). Monitoring Design

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Since 1989, the Cal Parisa basin has been provided with a pluviometric and hydrological network consisting of four rain recorders, two hydrometrical stations at each sub-basin outlet and one

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meteorological station (Fig. 1). Since 1992, soil water content profiles were instrumented both inside and outside forest patches with TDR sensors at 0-20, 20-40, 40-60 and 60-80 cm below the soil surface; readings were performed weekly. Shallow (20 cm) soil water content transects were also instrumented across the main geoecological units of the basin (Rabad~ et al, in press). Hydrological differences between vegetation covers were studied using grassed and forested experimental plots which were monitored form July 1993 onwards. The grass plot is located in an agricultural abandoned terrace covered by meso_phile grassland. A Bowen Ratio Energy Balance station, connected to a data logger that stores readings at five-minute steps, is used to monitor grassland evapotranspiration.

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Fig. 2. Monitoring design of the Ramon Poch experimental plot for the study of forest interception and transpiration. The forest plot (named Ramon Poch in order to bonour the memory of the second Author who suffered a fatal accident during a field campaign) is covered by a dense monospecific Pinus sylvestris stand (2800 stems haI) with very poor understorey. This plot is taken as representative of small patches of pines overgrown in marginal agriculture areas, which were abandoned in the first half of the Century. A data logger stores data from the following devices at five-minute steps (Fig.2): I) Three sets of three trough collectors, each one with a surface area of about 1 m 2, collect the throughfaU, which is then measured by three tipping buckets. 2) The stemflow received by 7 trees is also measured in three tipping buckets of the same design. 3) Six wetness sensors distributed within the canopy. 4) One large rainfall recorder (1 m 2 surface), with the same characteristics as the throughfall collectors, placed outside the forest area, together with a tipping bucket. 5) One conventional funnel rain recorder (200 cm~) with a resolution of 0.2 mm placed at ground level.

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Six trees are monitored to measure transpiration using Granier's (1985) sap flow measurement technique (Llorens & Pooh, 1994).

Experimental Results The following results are described in the following way: firstly dry conditions including forest and grassland transpiration data, and secondly wet conditions including forest interception data. Dry Conditions: Figure 3 shows the transpiration rates for pine and grass during both dry and moderately wet periods. The graph clearly demonstrates that transpiration is controlled better by forest lhan grassland, mainly during the drought period when the trees are able to reduce transpiration rate by half. Even with soil moisture deficits, grassland shows transpiration rates to be clearly dependent on radiative energy.

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Fig. 3. Comparison between forest and grassland transpiration during dry conditions. Ea-forest=Forest transpiration; Eagrass----Grassland Wanspiration; Rn=Net radiation (in water-equivalent); S-forest=Forest Soft water reserve, and S-grass---Grassland soil water reserve. Wet Conditions: The study of the rainfall-interception relationship at the event scale (Llorens et al., submitted) shows that interception losses by the forest patch represent about 29% of bulk rainfall. Relative interception decreased from more than 50% for events of less than 8 mm to about 20% for events greater than 20 mm, and remained constant for greater events. Interception depths increased with rainfall, following a curve with positive but decreasing slope (Fig. 4).

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Fig. 4. (Top of former page): Relationship between rainfall interception and bulk precipitation at the event scale.

Fig. 5. (Bottom of former page): Distribution of the studied events together with rainfall and other weather variables in the plane defined by the two first axes resulting from a Principal Component Analysis. D=event duration, P=rainfall, I=interception, Ip=rainfall intensity, T=air temperature, Vfwind velocity, VPD=water vapour pressure deficit, and I/P--relative interception. I and II represent the classes of events. From a multivariate analysis (Llorens et al.. submitted) events could be classified as follows (Fig. 5): Class I: Long events with low rainfall intensities producing lower interception rates (14%). Class II: Short events with high rainfall intensities and dry atmospheric conditions producing higher interception rates (32%). The presence of these two groups of events suggests: A process with low interception due to low evaporation rates that are efficient because of the length of time when the canopy is wet (Class I) and a process with high interception rates due to active reevaporation during rainfall (Class II).

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Water Balance: Figure 6 presents the spatial variability of surface soil moisture along a transect cutting the main geoecological units of the basin (Raba~ et al., in press), with the measures obtained respectively during dry and wet conditions. Looking at the driest transect it can be seen that, even if there are some differences between grass and forest behaviour, they generate only slight variations in the water balance. Contrarily, in the humid transect it can be clearly observed that forest patches represent negative anomalies of the spatial distribution of soil moisture, it being clear that rainfall interception overcomes differences in transpiration and results in strong differences in compared water balance. This

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behaviour is not only evidenced at the soil surface, but also clearly appears in the different soil moisture profdes from 0 to 80 cm depth.

SOME PROBLEMS IN THE GENERALIZATION OF THE INTERCEPTION PROCESS IN MEDITERRANEAN WEATHER CONDITIONS Review of Classical Models Until the early seventies attempts to generalize interception losses were always expressed in the form of empirical regressions between interception loss and bulk rainfall (for reviews see: Zinke, 1967; Helvey & Pattie, 1965). In contrast to this approach, Rutter et at. (1971; 1975) constructed the first physicallybased interception model working with rainfall and meteorological data inputs that are used to calculate the water balance of the canopy. This model has been successfully tested mainly in humid-temperate conditions and could be considered the most rigorous method to predict rainfall interception. As the Rutter model needs detailed rainfall and meteorological data to predict interception, Gash (1979) developed an analytical model that retained some of the simplifications of the early works based on regression equations and coupled them with the physically-based proposals of the Rutter model. The Rutter and Gash models are based on three main components: The bulk rainfall input. The parameters related to the canopy structure, like the canopy water capacity or the free throughfall coefficient. The evaporation of the intercepted water, based on the Penman-Monteith equation for wet surfaces (Monteith, 1965). These models have rarely been tested in Mediterranean conditions, where a significant part of the annual precipitation is produced during convective events characterized by high rainfall intensifies and relatively high water vapour deficits. From the three model components, and eliminating the rainfall that is the model input, the determination of the evaporation rate using Penman-Monteith equation is the only component that would depend on these Mediterranean-type conditions. The canopy parameters, as well as being difficult to determine, are not dependent on these conditions.

Validation of Predicted Penman-Monteith Evaporation Rates Following this reasoning the first step to test the validity of such classical interception models is to validate the prediction of interception rates using the Penman-Monteith equation. Figure 7 clearly shows that interception calculated using the Penman-Monteith equation underestimates interception losses, mainly in Mediterranean-type events (Llorens et al., 1995). The main reasons for this underestimation are due to the assumptions in its application, that can be summarized as follows: Typical Mediterranean convective events are far different from the assumed meteorological conditions in the interception models. These assumed meteorological conditions are related mainly to the assumption of neutral stability conditions and the subsequent logarithmic wind profile. In mountainous areas, where surface roughness not only depends on vegetation roughness, it is difficult to apply models built using wind conditions over flat areas.

CONCLUSIONS The main process modifying a basin water balance when cover changes from grass to forest is clearly rainfall interception that overcomes the effect of differences in transpiration.

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Present results on the rainfall interception process are consistent with findings in other study areas, but present some singularities due to the weather conditions during rainstorms. Difficulties in modelling interception processes are emphasized. Current models can underestimate interception because of the underestimation of evaporation rates of intercepted water. Extending experimental plots to different topographical and stand conditions is necessary. The density of trees and the size of the forest patch are perceived as the main aspects that can influence the present results. More research is also needed to solve problems that appear in physically-based modelling approaches. Finally, the present results, even if provisional, confirm the major importance of land cover on water balance, and demonstrate the need to improve the hydrological adequacy of land management policies.

ACKNOWLEDGEMENTS This work was financed by the DM2E (CE EV5V-CT91-0039) project funded by the EC and the AMB93-0806 funded by the CICYT. The contribution of the first author was supported by a post-doctoral research contract funded by the Direcci6n General de Investigaci6n, Ministerio de Educaci6n y Ciencia. The contribution of the second

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author was supported by the Water Programme (Programa Movilizador del Agua) from the CSIC. The contribution of the third author was supported by a pre-doctoral fellowship from the Departament d'Ensenyament de la Generalitat de Catalunya. The authors are indebted to Oscar Avila and J6r6me Latron for their help with instrumentation in the experimental plots and field work.

REFERENCES Bosch, J.M. & Hewlett, J.D. (1982): A review of catchment experiments to determine the effect of vegetation changes on water yield and evapotranspiration. J. Hydrol., 55: 3-23. Gallart, F.; Llorens, P. & Latron, J. (1994): Studying the role of old agricultural abandoned terraces on runoff generation in a Mediterranean small mountainous basin. J. Hydrology, 159: 291-303. Gash, J.H.C. (1979): An analytical model of rainfall interception by forests. Quar. J. Roy. Met. Soc., 105(443):43-55. Granier, A. (1985): Une nouvelle m6thode pour la mesure du flux de s~ve brute dans le tronc des arbres. Ann. Sci. For., 42(2): 193-200. Helvet, J. D. & Patric, J.H. (1965): Canopy and litter interception of rainfall by hardwoods of eastern United States. Water Resour. Res., I: 193-206. LLorens, P. (in press): Hydrological implications of Mediterranean Mountainous Land Abandonment: Findings and Questions Arising from the Research in a Small Basin (Cal Parisa, Eastern Pyrenees). Acta Geol. Hispanica, Special Issue: Assessing Hydrological Changes. LLorens, P. & Gallart, F. (1992): Monitoring a Small Basin Response in a Mediterranean Mountainous Abandoned Farming Area: Research Design and Preliminary Results; Catena 19: 309-320. LLorens, P.; Latron, J. & Gallart, F. (1992): Analysis of the role of agricultural abandoned terraces on the hydrology and sediment dynamics in a small mountainous basin (High Llobregat, Eastern Pyrenees). Pirineos, 139: 27-46. Llorens, P. & Poch, R. (1994): Monitoring of water losses to the atmosphere at the Cal Parisa Basin. In: Conference on Assessment of Hydrological Temporal Variability and Changes. Field Excursion Guide. EJC.B., Barcelona, Spain.15-22. Liorens, P.; Poch, R. & Gallart, F. (1995): Non-Micrometeorological monitoring of water fluxes to the atmosphere from a pine stand in a Mediterranean mountainous area. Annales Geophysicae, Vol. 13, Supp. 2: c505. Llorens, P.; Poch, R. & Gallart, F. (submitted): Rainfall interception by a Pinus sylvestris forest patch overgrown in a Mediterranean mountainous abandoned area. Monitoring design and results down to the event scale. J. Hydrol. Miller H.G. (1992): Forest response to changes in land use and implications for ecosystem functioning and management. In: Teller, Mathy & Jeffers (Eds): Response of forest Ecosystems to environmental changes, Elsevier. 555-559. Monteith, J. L. (1965): Evaporation and environment. In: State and movement of water in living organisms. Symposium Society Experimental Biology, 19: 205-234. Rabad~, D.; Gallart, F. & Llorens, P. (in press): Monitoring soil moisture variability in the Cal Parisa basin (Alt Llobregat) with TDR: Experimental design and first results. Acta Geol. Hispanica, Special Issue: Assessing Hydrological Changes. Rutter, A.J. (1967): An analysis of evaporation from a stand of Scots pine. In: Sopper & Lull (Eds): Symposium on Forest Hydrology, Pergamon Press, Oxford: 403-417. Rutter, A.J.; Kershaw, K.A.; Robins, P.C. & Morton, A.I. (1971): A predictive model of rainfall interception in forest. 1. Derivation of the model from observations in a plantation of Corsican pine. Agric. Meteorol., 9: 367-384. Rutter, A.J.; Morton, AJ & Robins, P.C. (1975): A predictive model of rainfall interception in forest. 2. Generalization of the model and comparison with observations in some coniferous and hardwood stands. J. Appl. Ecol., 12: 367-380. Zinke, PJ. (1967): Forest interception studies in the United States. In: Sopper, W. E. & Lull, H.W. (Eds): Symposium on Forest Hydrology. Pergamon Press, Oxford: 137-161.