Components of the energy and water balance at the HAPEX-Sahel southern super-site

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1 Exchange Processes al the Land Surface/or a Ranee of Space and Time Scales (Proceedings of the Yokohama Symposium, July 1993). IAHS Publ. no. 212, Components of the energy and water balance at the HAPEX-Sahel southern super-site J. S. WALLACE, S. J. ALLEN, J. H. C. GASH, C. J. HOLWILL & C. R. LLOYD Institute of Hydrology, Wallingford, Oxfordshire X1 8BB, UK Abstract This paper describes measurements of the total and component fluxes of heat and water vapour from sparsely vegetated land surfaces in the Sahel. The work presented was undertaken by the Institute of Hydrology as part of HAPEX-Sahel, an international study of the energy, water and carbon balances of a semiarid area in Niger. The measurements reported were made at two of the "sub-sites" (fallow savannah and tiger-bush) constituting the southernmost of the three HAPEX-Sahel "super-sites", located within the one degree square centred on Niamey. Total and component fluxes were measured by eddy correlation, Bowen ratio energy balance, sap flow gauge and chamber techniques. Some preliminary results from each of these techniques are presented to illustrate the key factors controlling the energy and water balance of the two vegetation types. INTRODUCTION HAPEX-Sahel is an international study of the large scale energy, water and carbon balances of a semiarid area in Niger, West Africa (Goutorbe et al., 1993). The objective of the experiment is to improve the parameterization of semiarid land surface processes in general circulation models (GCMs) because of the possible link between land degradation and climatic change in these regions. Several modelling studies have indicated that the removal of vegetation may modify the surface energy balance in such a way as to reduce rainfall (e.g. Charney, 1975; Walker & Rowntree, 1977; and Cunnington & Rowntree, 1986), which would have serious consequences for agricultural production (Sivakumar, 1992). However, the forecasts of GCMs are very sensitive to the way in which the land surface conditions are prescribed. For example, in a recent simulation of the Sahelian climate Xue et al. (199) clearly demonstrated the importance of incorporating a physically realistic representation of vegetation in GCMs. Unfortunately very little data are currently available to derive the necessary parameterization s of Sahelian vegetation. There is therefore an urgent need for accurate information on the energy and water balance of typical land surfaces in the Sahelian environment. This paper describes some of the work carried out by the Institute of Hydrology during the HAPEX-Sahel intensive observation period (IOP) which took place from 17 August to 9 October We present brief descriptions of the methods used to measure the total and component fluxes of heat and evaporation in the fallow savannah and tiger-bush sub-sites. Some preliminary results are also presented to illustrate the key factors controlling the energy and water balance in this semiarid area.

2 366 J. S. Wallace et al. MATERIALS AND METHODS Sites and vegetation During the IOP there were three super-sites, one of which, the southern super-site, was located about 45 km south of Niamey, Niger around the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) Sahelian Center (ISC). The area consists of a broad sandy valley next to the river Niger bordered to the west by extensive latérite plateaux. The vegetation on the plateaux is known as tiger-bush, on account of its striped pattern. The remainder of this area is predominantly agricultural land, containing either millet fields or fallow savannah. The fallow site, located 4 km west of the ISC ( 'N, 'E), had not been cropped for 6 years, allowing the natural vegetation to regenerate. The area measured approximately 8 x m and was covered by a ground layer of annual herbs and grasses interspersed with 2-3 m tall shrubs (almost entirely Guiera senegalensis L.) and occasional 4-8 m tall trees. The soil on the site was sandy, about 3 m deep. The tiger-bush site, was located 6 km south-west of ISC ( 'N, 'E) within an irregularly shaped plateau about 3 km across. The vegetation is confined to dense bands about 1-3 m wide by -3 m long separated by completely bare crusted soil. About 33% of the area was occupied by the vegetated strips dominated by two species of 2-4 m tall shrubs (Combretum micranthum G. Don and Guiera senegalensis L.) and a single tree species (Combretum nigricans Lepr. ex Guill. and Perrott.) typically 4-8 m tall. The soil on the site is a gravel with a fine to coarse sandy matrix,.2 to.5 m deep, overlying latérite rock. Further details of the site are given by Culf et al. (1993). The climate is typical of the southern edge of the Sahelian zone with summer rainfall and high temperatures throughout the year. Annual Penman (1948) potential evaporation at Niamey is nearly four times the mean annual rainfall of 562 mm (Sivakumar, 1987). During the 1992 season total rainfall was above average and well distributed throughout the season. The last rainfall during the IOP fell on 15 September. Above canopy fluxes Hourly fluxes of total evaporation and sensible heat from the two vegetation types were measured using a Mk 2 Hydra eddy-correlation device (Shuttleworth et al., 1988). At the fallow site the Hydra was mounted on a tower at a height of 9.5 m above the ground, with a southerly fetch of about 4 m. The Hydra at the tiger-bush site was mounted on another tower at 17.8 m, with a fetch of at least 1 km in all directions. Net radiation at the tiger-bush site was measured using two net all wave radiometers (REBS Q*6, Seattle, USA) mounted separately over the bare soil and vegetation strips. The mean net radiation for the entire site was taken as the area weighted mean of the fluxes from these two instruments.

3 Bare soil evaporation HAPEX-Sahel energy and water balance components 367 Fluxes of evaporation and sensible heat over the tiger-bush bare soil were measured independently using a small modified Bowen ratio system (Campbell Scientific Ltd., UK). This system measured temperature and humidity at two levels (.5 and.2 m) above an area of completely bare soil where the fetch in a southerly direction was about 6 m. Temperature difference between the two levels was measured using a pair of differential thermocouples and air samples from the two levels were ducted to a common dew-point hygrometer. The system also had its own net radiometer mounted at 25 cm above the soil surface and two flux plates each with a pair of thermistors to measure soil heat fluxes. All these data were recorded at 2 minute intervals with a solid state logger (21X - Campbell Scientific Ltd., UK). Woody shrub transpiration Transpiration from the woody shrubs at the fallow and tiger-bush sites was measured using constant power sap flow gauges (Dynamax, Houston, USA) as described by Baker & Van Bavel (1987). At the fallow site eight stems of G senegalensis were fitted with sap flow gauges and their output recorded every 1 minutes with a solid state logger (21X - Campbell Scientific Ltd., UK). At the tiger-bush site a similar system was used to monitor sap flow in 6 stems of G senegalensis along with a further eight stems of C. micranthum. Fallow herbs conductance On a number of days the conductance of the herbaceous layer at the fallow site was measured using an evaporation chamber similar to that described by Kohsiek (1981). On measurement days ten sample areas (.6 x.44 m) of the herbaceous substrate were isolated using metal borders pushed into the soil. A large transparent chamber (.44 m high) was placed on these locations at approximately 2 hour intervals during the day and the rate of increase in temperature and humidity over a 2 minute period was recorded with a solid state logger (21X - Campbell Scientific Ltd., UK). These data were then used to calculate the surface conductance of the soil and plants enclosed by the chamber. RESULTS AND DISCUSSION Figures 1(a) and (b) show a comparison of the total fluxes of evaporation (LE), sensible heat (H) and net radiation (R ) for the tiger-bush site over two different days during the IOP. On 16 September the surface was wet following a small rainstorm (3 mm) the previous day. At this time evaporative fluxes dominated, reaching 4 W m" 2 around midday when sensible heat fluxes were only ~ 15 W m" 2. No rain fell during the following two weeks and so on 1 October evaporation rates had decreased to 3 W mf 2 around midday while sensible heat fluxes had increased to 25 Wnr 2.

4 368 J- S. Wallace et al. Wet soil Tiger-bush Dry soil STh s X y* U) m h- 7nn boo son 4 MOO?oo (a) 16 Sep R (b) 1 Oct E 5 lu S c oil in 7. (c) 16 Sep (d) 1 Oct i: F c n (e) 21 Sep (f) 1 Oct 1 SO 1-5 Fig. 1 Energy fluxes over tiger-bush during two days with wet and dry soil: (a) and (b) total evaporation (LE), sensible heat flux (H) and area average net radiation (RJ; (c) and (d) soil evaporation (LE S ), sensible heat flux (H s ) and bare soil net radiation (R ns ); (e) and (f) transpiration from G. senegalensis (G.s.) and C. micranthum {Cm.). The contribution of the bare soil patches in the tiger-bush to the area average energy balance is shown in Figs 1(c) and (d). On the day when the soil had been recently wetted, there was significant soil evaporation (LE S ) during the morning. However, this rapidly declined during the day, with a consequential increase in the bare soil heat flux (H s ). The morning LE S fluxes made a significant contribution to the total evaporative flux from the entire area (LE) and this is reflected in the high values of LE recorded by the Hydra at this time (Fig. 1(a)). Under the very dry conditions experienced on 1 October there was very little soil evaporation and the bare soil heat flux had increased to -25 W m" 2 at midday. At this time, therefore, the total evaporative flux (LE) was almost entirely from the vegetation strips. The rapid drying of the soil observed here has also been reported for sandy soil under millet in this environment (Wallace et al. 1992). Figures 1(e) and (f) show a comparison, of the mean transpiration per unit leaf area in G. senegalensis and C. micranthum at the tiger-bush site over two different days during the IOP. No sap flow measurements were made on 16 September, so Fig. 1(e) shows data for 21 September, the first day when the gauges were operational.

5 HAPEX-Sahel energy and water balance components 369 Wet soil Fallow savannah Dry soil (a) 26 Aug (c) 26 Aug (b) 1 Oct (d) 1 Oct (e) 26 Aug GMT (f) 1 Oct E Fig. 2 Energy fluxes over fallow savannah during two days with wet and dry soil: (a) and (b) total evaporation (LE), sensible heat flux (H) and area average net radiation (R n ); (c) and (d) surface conductance of the herbaceous substrate; (e) and (f) transpiration from G. senegalensis (G.S.). Transpiration rates in G. senegalensis were about twice those in C. micranthum at this time. Ten days later, on 1 October (Fig. 1(f)), transpiration rates in both species had declined markedly, the difference between them remaining a factor of two. Total fluxes of evaporation, sensible heat and net radiation at the fallow site on two different days of the IOP are shown in Figs 2(a) and (b). Very high evaporation rates were observed in August when the surface was wet and the vegetation had its maximum leaf area. By 1 October, 2 weeks after the last rains, evaporation rates had markedly decreased, but were still greater than concurrent sensible heat fluxes. Evaporation rates at the fallow site on 1 October were similar to those measured on the tiger-bush site on the same day (cf. Figs 1(b) and 2(b)). Figures 2(c) and (d) show the diurnal pattern of the conductance of the herbaceous substrate at the fallow site on two contrasting days. On 26 August the soil surface was wet following 4.5 mm rain the previous day and the measured surface conductances of the combined soil/herb mixture were ~9 mm s" 1 in the morning. Surface conductance decreased steadily throughout this day, presumably as a result of the soil surface drying. When the soil surface was dry, as on 1 October (Fig. 2(d)), surface conductances were low all day at 2-3 mm s" 1. GMT

6 37 /. S. Wallace et al. The mean transpiration per unit leaf area in G. senegalensis at the fallow site at two different times of the IOP is shown in Figs 2(e) and (f). The diurnal pattern of transpiration was closely coupled to net radiation, even showing a response to the short term fluctuations in R n observed in the afternoon of 26 August and the morning of 1 October. Measured transpiration rates were higher on the 1 October than earlier in the season, the opposite to the behaviour observed at the tiger-bush site. This may have been caused by the different soil moisture reserves available at the two locations. The fallow site bushes may have had access to sufficient soil moisture on 1 October so that the bushes were able to respond to the drier atmosphere at this time by transpiring faster. At the tiger-bush site the soil moisture storage may have been limited by the shallow soil depth, so the higher atmospheric demand in October could not be met and the transpiration rate was limited by soil water supply. CONCLUDING REMARKS The results presented in this paper are preliminary and a much fuller analysis will be required before more detailed assessments of the relationships between the component and total fluxes can be made. However, we have demonstrated that it is possible to measure the component and total fluxes of energy in these complex vegetation types. Furthermore, an initial inspection of the data indicate that the measurements behave in a consistent manner, giving confidence in their quality and suitability for further analyses. With the information obtained we can now proceed with the exciting task of developing models which can be used to predict the behaviour of these Sahelian land types. Ultimately these models will be incorporated into GCMs which should provide a more credible tool for understanding the processes of climate change and land degradation in the semiarid tropics. Acknowledgements We acknowledge the financial support provided by the UK Overseas Development Administration and the UK Natural Environment Research Council through its TIGER (Terrestrial Initiative in Global Environment Research) programme, award number GST/91/III.1/1A. We are also grateful to all those members of the staff of the ICRISAT Sahelian Center and the Institute of Hydrology who contributed to the success of this work. REFERENCES Baker, J. M. & Van Bavel, C. H. M. (1987) Measurement of mass flow of water in the stems of herbaceous plants Plant Cell Environ. 1, Charney, J. G. (1975) Dynamics of deserts and drought in the Sahcl. Quart. J. Roy. Mel. Soc. 11, Culf, A. D., Allen, S. J., Gash, J. H. C, Lloyd, C. R. & Wallace, J. S. (1993) The energy and water budgets of an area of patterned woodland in the Sahel. Agric. For. Met. (in press). Cunnington, W. M. & Rowntrce, P. R. (1986) Simulations of the Saharan atmosphere: dependenceon moisture and albedo. Quart. J. Roy. Met. Soc. 112, Goutorbe, J. P., Lebel, T., Tinga, A., Bessemoulin, P., Brouwcr, J., Dolman, A. J., Engman, E. T., Gash, J. H. C, Hocpffner, M., Rabat, P., Kerr, Y. H., Monteny, B., Prince, S., Said, F., Sellers, P. & Wallace, J. S. (1993) HAPEX-Sahel: A large scale observational study of land-atmosphere interactions in the semi-arid tropics. Annales Geophysicae (in preparation).

7 HAPEX-Sahel energy and water balance components 371 Kohsiek, W. (1981) A rapid circulation evaporation chamber for measuring bulk stomatal resistance. J. Appl. Meteorol. 2, Penman, H. L (1948) Natural evaporation from open water, bare soil and grass. Proc. Roy. Soc. A193, Shuttleworth, W. J., Gash, J. H. C, Lloyd, C. R., McNeill, D. D., Moore, C. J. & Wallace, J. S. (1988) An integrated micrometeorological system for evaporation measurement. Agric. For. Met. 43, Sivakumar, M. V. K. (1987) Climate of Niamey. Progress Reporl-1. ICRISAT (International Crops Research Institute for the Semi-Arid Tropics) Sahclian Center, Niamey, Niger. Sivakumar, M. V. K. (1992) Climate change and implications for agriculture in Niger. Climatic Change 2, Walker, J. & Rowntree, P. R. (1977) The effect of soil moisture on circulation and rainfall in a tropical model. Quart. J. Roy. Met. Soc. 13, Wallace, J. S., Lloyd, C. R. & Sivakumar, M. V. K. (1992) Measurements of soil, plant and total evaporation from millet in Niger. Agric. For. Mel. (in press). Xue, Y., Liou, K.-N. & Kasahara, A. (199) Investigation of the biogeophysical feedback on the African climate using a two-dimensional model. J. Climate, 3,

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