Aff orestation and W ater Supplies in Britain
By L. Leyton and A. Carlisle,
Imperial Forestry Institute, University of Oxford
In 1956, at a meeting of the British Waterworks Association, the Minister of Hou- sing and Local Government, the Rt. Hon. Duncan Sandys, M. P. stated «it is certain that the demand for water in the future will be on a much greater scale than anything we have had so far ... it is perfectly plain that the price, the quantity available and the quality of water supplied in the area concerned will be an increasingly important factor in deciding where our manufacturing industries are going to set up new works and new factories» (Sandys, 1957). The difficult situation regarding Britain's water supplies was further emphasized in 1957, in an editorial in the «Economist», pointing out «that although we have an annual precipitation of about 48" ( 1220 mm), the growing demands from industry borne by some water undertakings are causing a certain amount of alarm, particulary where marginal capacity of existing supply and drainage arrangements is inadequate for further development» (Anon.1957). lt has been estimated that Britain's demands on water supplies are increasing by about 2
%
per annum so that unless some- thing can be done about the problem, it will not be many years before the situation be- comes quite critical. These statements reflect a situation which is by no means uncom- mon in other European countries; e. g. Friedrich (1954) has already drawn attention to similar problems in Germany. Nor does this situation come entirely as a surprise since for many years, at least in certain heavily populated districts in Britain, water has had to be carefully rationed during spring and early summer drought periods.Not only are the water engineers concerned in this problem but also the foresters.
lt has been the policy of the British Forestry Commission, since its inception in 1919, to build up Britain's timber supplies, severely depleted by centuries of deforestation. As a result, the past few decades have seen the growth of new forests at an impressive rate, especially in the important catchment areas of Wales and of Scotland. For a long time this policy has met with general approval but in recent years, with the increased aware- ness of the insufficiency of our water supplies, water engineers have begun to question the wisdom of this policy. Foremost among the critics has been Mr. F. Law, of the Fylde Water Board who, in 1956 (Law, 1956) published a paper based on lysimeter investi- gations in which he claimed that afforestation of 1500 acres of moorland with spruce would result in a daily loss of one million gallons of water. He quotes «if the figures ...
are ... proved to be true, it will be up to the Forestry Commission, despite their legal right, to help to repair the damage already done by afforestation; whether this will mean complete felling over !arge areas, leaving only belts of trees to give shelter, or the planting of different species such as larch, instead of spruce, is a problem for the future».
«Mitteilungen der Schweizerischen Anstalt für das forstliche Versuchswesen, Bd. 35, Heft l»
This viewpoint not unnaturally raised a considerable amount of public interest and concern and although Law's estimates can be criticized from several points of view, especially regarding the general reliability of_ his technique and the small area on which his extrapolations are based, they have nevertheless received some attention be- cause, to date, they are the only directly obtained quantitative data available in Britain.
lt is in fact remarkable that so little quantitative work has been clone in this country on the effect of a forest cover on water supplies. No doubt this has been due partly to the relatively small area of Iorest and partly to a general Jack of a sense of urgency, en- couraged by the past assumption that Britain is plentifully supplied with water and the apparent absence of serious flooding and erosion. There has, of course, been a consi- derable amount of work on hydrological relations in general; the work of Penman on evapotranspiration, for example, has received world wide recognition. Unfortunately this valuable approach is of limited application to forest crops on catchment areas since it assumes a short crop under conditions of non-limiting water supply (i. e. potential rather than actual evapotranspiration). Nevertheless the following data on estimated potential evapotranspiration values for Britain are of interest (cf. Penman, 1950);
Estimated potential Average precipitation
Region evapotrnnspiration
inches per annum inches per annum
N. W. Scotland 14~15 50-80+
E.Scotland 15-16 25-30
Wales 16-20 4,0-80
S. W. England 18-24, 4,0-50
S. E. England 18-20 25-4,0
E. England 14~17 20-30
These figures suggest at least an upper limit Lo the order oI the losses to be expected from a forest stand and, taking into account the annual rainfall, point to the particularly difficult situation in the Eastern part of the country. Of more direct interest to those concerned with forest influences are the investigations of Ovington and of Rutter.
Ovington (1954,) has measured interception and stem flow in plots of various tree spe- cies and though his technique must involve considerable sampling errors, his results sug- gest that interception can be a very important Iactor in the water balance oI the forest;
over the period 1949-1951, the mean annual values ranged from about 50 % ( under Pseudotsuga taxifolia) Lo about 20
%
(under Larix decidua) but extremes of from 6 to 93%
were observed on particular occasions. Stern flow in these species was found to constitute only a very small proportion of the total precipitation reaching the forest floor; an important observation which will be discussed in more detail later, was that the tree canopies induced the formation of !arge droplets resulting in a very irregular distribution of water on the forest floor. Rutter (1957) has approached the problem from a more physiological point of view and, using the rapid weighing method with cut shoots from different regions of the crowns, and extrapolating the results to the whole stand, has estimated total transpiration of a young ( 16 year old) stand of Pinus sil-Figure 1
4
:
-5 8 0 0 8 0 0.s 3 0 0
e 0 8 0
"'
il 0
~2 0
-5 81
.; "il'
~
4 5 6
Distance of gauge from stem lfeet)
Relation between location of rain gauge under the tree canopy and total catch
during August, 1958
60
"
g40l i20
~10 0
0
• 10 0
Figure 2
0 0 0
0
0 0
0 0
0 0
0 0 0 0
0
0
0 0
2 4 5 6 7 8
Oaily hours of rainfall
Relation between daily hours of rainfall and daily interception by the tree canopy
vestris; du ring the summer and autumn months this amounted to about 3 inches per month. For this stand he found no evidence for a decrease in transpiration even when the soil moisture deficiency exceeded 6 inches.
More recently, work in this field has begun at the Forestry Department at Oxford.
In a preliminary study, primarily concerned with the development of suitable methods and techniques, investigations have been made on interception and stem flow in a young (c. 22 year old) stand of Chamaecyparis l,awsoniana. Data presented in figure 1, show that the total catch, over a given period, oI gauges placed at different distances from the stem of a single tree, varies considerably: this observation confirms the extreme care necessary in placing gauges beneath tree crowns for the determination of interception.
In this particular case, through-fall increased regularly with increasing distance from the stem; the wide scatter at the edges of the crown is more or less to be expected be- cause of irregular drip, the swaying of the crown in the wind and the access of rain coming in at an angle. lt is also worth noting that gauges situated at the S. W. edge of the crown i. e. on the side of the prevailing rainbearing winds gave higher than average catches. Average interception by the whole crown was estimated by calculating the mean catch at different distances from the stem and weighting these means according to the area represented. As an example, during the month of August 1958, with a total precipi- tation, in the open, of 3.3 inches an average interception of about 18
%
was obtained.Naturally interception will vary according to the intensity and distribution of the rain- fall apart from the situation of the gauge; this is clearly illustrated in figure 2 which reveals a distinct fall in the daily values for interception with increasing duration of rainfall in the previous 24 hours. The observation that occasionally, average through- fall estimates may exceed the rainfall in the open can be attributed to local concentra- tions of precipitation by the crown apart from experimental errors. For this particular stand it has been estimated that canopy saturation occurs with a rainfall of approxima- tely 0.2 inches, after which interception is negligible. Measurements oI stem flow con- firm previous findings that even during mist when the values show a distinct rise, this contributes relatively insignificant amounts to the water reaching the forest floor.
lt is scarcely necessary to emphasize that interception of rainfall by Lree canopies represents only one component of the water balance of the stand; nor does this repre- sent a total loss to the soil since it might be argued that such losses may be compensated by reduced transpiration when the foliage is saturated.
As far as !arge scale investigations on British catchment areas are concerned, there appears to have been only one which has any bearing on the effect of a forest cover;
Lloyd (1953) has compared hydrographs for two catchment areas in Wales, similar in most respects except that one has approximately a 3
%,
and the other a 60%
forest cover. He observed that after isolated storms, the peak discharge from the latter was less than half of that of the former but the period of discharge was appreciably longer: however after a succession of short storms, the final peak discharge from the more fo- rested catchment was as high as that from the treeless site from which it follows that a forest cover does not influence the intensity of extreme peak floods so much as reduce their number. With the increased interest in these problems, further investigations on catchment areas will no doubt be made (cf. e.g. Le~is, 1957) and though the initiative~ at present lies largely with the water engineer, foresters are bound to become increa- singly involved.Thus the situation in Britain Lo date is that interest and concern in forest hydrologi- cal problems is rapidly expanding but in view of the relativcly small amount of experi- mental data available, we have at present to look to othcr countries for inspiration. In this respect the classical investigations carried out by Burgcr and his colleagues of the Swiss school in the now famous Rappengraben and Sperbelgraben occupy a unique position. Despite differences in climate and topography, Burger's publications have had a considerable influence on the British approach to water management problems. The values he reports for evapotranspiration (namely of the order of 33" from the weil stocked catchment and 27" from the poorly stocked catchment) are somewhat higher than would be expected under British climatic conditions but his observations on the role of the forest as a regulator of water supplies are of considerable significance in Bri- tain. lt is of particular interest therefore that Lloyd's findings referred to above, con- firm Burger's earlier observations and no doubt British foresters will have much to gain by emphasizing this point. As yet we have little information on the effect of a tree cover on soil moisture properties and though it is often claimed that such cffecls are slow to develop, Ovington (1956) has obscrvcd differences in porosity bctween different tree covers within as liLLle as 20---40 years f rom establishmenl. Thc findings of Burger and his colleagues have slressed the necd ror similar investigations under Brilish conditions and, by pointing out these importanl fcalurcs, the Swiss school has provided a stimulus for which we shall long be indebted.
Aufforstungen und Wasserversorgung in England und Wales
(Zusammenfassung)
Die künftig sich noch verschärfende Wasserknappheit immer größerer Gebiete in Engl.and und Wales hat weite Kreise alarmiert und unter anderem auch eine Kontro- verse über den Einfluß umfangreicher Aufforstungen auf den Wasserhaushalt hervor- gerufen. Während die Ergebnisse einer ersten, allerdings umstrittenen Untersuchung eine negative Wirkung von Aufforstungen auf den Wasserhaushalt zeigten, ergab ein zweiter Versuch ähnliche Resultate wie im Emmental, die auf eine positive Wirkung von Aufforstungen auf den Wasserhaushalt hinwiesen. Der Autor betont die Bedeutung ver-
mehrter Untersuchungen der Wasserbil.anz ganzer Einzugsgebiete. Er bespricht ferner die Ergebnisse von Untersuchungen über lnterzeption und Schaftwasser, zwei noch rel.ativ wenig erforschte Faktoren des W asserkreisl.auf s.
Les reboisements et l'approvisionnement en eau en Grande Bretagne et au Pays de Galle
(Resume)
On s' est al.arme dans certains milieux de /,a penurie d' eau qui se fait sentir d'une faqon toujours plus accusee dans de vastes regions de l' Ang.leterre et du Pays de Galle.
A cette occasion, l'influence des travaux de reboisement sur le regime des eaux a ete fortement controversee. C'est ainsi qu'a la suite de premiers travaux de recherches, dont les resultats sont du reste contestes, on a conclu a une influence defavorable des reboise- ments sur le regime des eaux. De nouvelles recherches ont, par contre, donne des resul- tats analogues a ceux obtenus dans l' Emmental, qui font ressortir l'influence favorable des reboisements sur le bilan des eaux. L'auteur souligne l'importance de poursuivre activement les recherches en matieres d'hydrologie en les etendant a la surface totale des bassins de reception. Il traite, en outre, des resultats de nouveaux travaux de re- cherches sur les quantites d' eau interceptees par /,a cime des arbres ainsi que celles qui s' ecoulent le lang du tronc.
Rimboschimenti e rifomimento di acqua in Inghilterra e in Galles
(Riassunto)
La scarsita di acqua, ehe in regioni sempre piu vaste dell.a Bretagna diventa in pro- gresso di tempo sempre piu acuta, ha all.armato vaste cerchie e suscitato tra altro una
controversia circa l'influsso di vaste opere di rimboschimento sul regime idrico. Cos'i, una contestata indagine, ha messo in risalto un influsso f ortemente negativo ehe i rim- boschimenti eserciterebbe sul regime idrico. Questo risultato reclama delle ricerche piu intense sul bilancio idrico di interi bacini imbriferi, con la misurazione dei quantitativi del deflusso. L'unico lavoro del genere eseguito finora nel Galles ha dato risultati ana- wghi a quelli rilevati dal Burger nell' Emmental.
N ell' A rticolo si discute inoltre i risultati di nuove ricerche sulla intercezione (l' acqua trattenuta dalla vegetazione) e sull'acqua ehe scola lungo i fusti, cioe su due fattori an- cora poco conosciuti del ciclo idrico.
References
An o n, 1957: Is water short? The Economist, 26 th January. Also in J. Brit. Waterwks. Assoc., 39, 52-4.
Friedrich, W., 1954,: Wald und Wasser. Mitt. Arbeitskreises «Wald und Wasser». Nr. 1, 1-5.
La w, F., 1956: The effect of afforestation upon the yield of water catchment areas. J. Brit. Water- works Assoc., 38, 489-494.
Lew i s, W. K., 1957: Investigation of rainfaJI, run-off, and yield on the Alwen and Brenig catch- ments. Proc. Inst. Civ. Engrs., 8, 17-52.
LI o y d, D., 1953: Contribution to the discussion of Dobbie, C. H., and Wolf, P. 0., «The Lyn- mouth flood of August 1952». Proc. Inst. Civ. Engrs. (1953), 522-588.
0 v in g t o n, J. D., 1954: A comparison of rainfaJI in different woodlands. Forestry, 27, 41-53.
- 1956: Studies of the development of woodland conditions under different trees, IV. The ,ignition loss, water, carbon and nitrogen content of the mineral soil. J. Ecol., 44, pp. 171-179.
P e n man , H. L., 1950: Evaporation over the British Isles. Quart. J. Roy. Met. Soc., 76, 372-383.
Rutte r, A. J., 1957: Preliminary observations on the water balance of a plantation of Pinus sil- vestris. A paper read to the Society for Experimental Biology. 18. 12. 57.
Sand y s, Rt. Hon. Duncan, 1957: J. Brit. Waterwks. Assoc., 39, 13-14.
