Impacts of Forestation and Land Use on Water ...

Impacts of Forestation and Land Use on Water Balance and Low Flows in Northwest Germany Hartmut Wittenberg University of Lueneburg Suderburg, GERMANY

Abstract The extensive transformation of North Germany's heathlands into forests and agricultural areas since 1860 had a strong influence on the water balance of the Lunenburg Heath (Lüneburger Heide). Within a hundred years, forest areas, mostly planted with pine trees, increased from about 15 to more than 35 percent of the total area. Due to interception and transpiration by trees, the water consumption of forests is significantly higher than that of cultivated and fallow lands, causing a reduction of groundwater recharge and runoff formation. Time series analysis of water levels recorded since 1888 at the gauging station of Bienenbüttel, Ilmenau River, revealed a decreasing tendency of discharges since the beginning of the 20th century, significantly correlated with the growth of forest areas. Particular impact was found on low flows and flows during the summer season. Since about 1960 forest areas remained practically unchanged. However, a further reduction of baseflow and low flows continued due to increasing groundwater abstractions mainly for sprinkler irrigation purposes. The analysis of the historical time series of water level records allowed the identification of changes in the water balance. While the average annual precipitation of about 700 mm remained unchanged, average annual runoff height of the Ilmenau basin decreased from about 260 mm to less than 190 mm within a hundred years. About 40 mm of this reduction were caused by forest development while 30 mm are due to groundwater abstraction. Groundwater recharge and runoff of forest areas with about 75 % of conifers was determined to 60 to 100 mm while non-forested areas yield about 300 mm. The effective water consumption of forests was even significantly higher than that of sprinkler irrigated fields. A further forestation would thus reduce groundwater recharge and low flows in the rivers. A change from pines and spruces to deciduous trees would moderate this effect. Keywords: hydrology, heath, forestation, land use, groundwater abstraction, irrigation, low flow, baseflow separation, groundwater recharge, NW Germany

Landscape development and forestation in the Lunenburg Heath The heather (calluna vulgaris) vegetation which covered most of the Lunenburg Heath (Lüneburger Heide) region in Northwest Germany in the 19th century is today only preserved in some remaining spots, e.g. on military training areas. The heath resulted from the removal of original deciduous woods of predominantly oaks, beeches, maples and birch trees for the cultivation of land, for construction and fuel purposes and, since the Middle Ages, for the fire wood requirements of the Lunenburg salt works. No trees were planted and the extensive pasturage by sheep, cattle and pigs prevented the recovery of forests. Many agricultural areas lost their fertility after some time of exploitation and were given up. The heather plant covered soon the resulting badlands of predominantly glacious, sandy soils. Grazing by sheep of a regional breed (Heidschnucken) regenerated the heath and prevented the growth of competing plants. However, the frequent cutting of the upper humus and heath layer to use it as spread in stables, fertiliser or construction material endangered even this modest vegetation, leaving sand surfaces (podsol) at many places. The introduction of artificial fertilizers and machinery, e.g. deep ploughing, in the second half of the 19th century allowed the cultivation of many heath areas. Also, the value of forests for economy and nature was recognized. Since 1877, the systematic reforestation of heath lands, particularly with pine trees was supported by government premiums. Leuschner and Immenroth (1994) report about the dramatic change of landscape. Fig. 1 shows the development of forest areas in the Lunenburg heath (districts of Lunenburg and Uelzen/ catchment of the Ilmenau River). Covering less than 15% of the total area in 1878, forest areas grew to more than 30% in 1939. After a slight decrease caused by side effects of World War II, a further growth to nowadays about 36% took place. In these forests, the ratio between conifers (pines, spruces) and

deciduous trees of about 3:1 is quite different from that of the original autochthon wood habitats. It is only in recent times that deciduous or mixed forests are planted. Forestation was only possible by keeping away of sheep, especially of the regional breed "Heidschnucke" (Reimers, 1997). Indeed, the drop down of the sheep population in the district is strongly correlated with the increase of forest areas in this time period (Fig. 1). The conversion of heath lands to forests is also demonstrated in Fig. 2 showing the situation in 1898 and 1994 as documented by the respective editions of the topographic map TK 25, sheet 3128, village of Hösseringen.

40

400

forest

forest areas %

300 30

District Lunenburg 200

20 100

sheep population * 1000

District Uelzen

sheep 10

1870

1890

1910

1930

1950

1970

0 1990

Figure 1. Development of forest cover and decrease of sheep population in the Lunenburg Heath

Figure 2. Heath and forest areas in 1898 (left) and 1994 (right), village of Hösseringen, Lunenburg Heath, North-Germany

Water levels and flows at the gauging station of Bienenbüttel/ lmenau River The catchment of the Ilmenau River at the gauging station of Bienenbüttel with an area of A = 1434 km2 is practically identical with the district (Landkreis) of Uelzen. Daily water levels are available since 1888. Due to the influence of seasonally varying waterweeds a steady rating curve could not be established. Therefore, daily flows are determined only since 1955 using a special approach (η method, Gils, 1962) with monthly current metering and calibration. In this study, an attempt was made to evaluate the time series of 115 years of daily readings despite of some deficiencies and uncertainties. Some corrections had to be carried out: Until 1950, the gauging station was situated at the railway bridge over the Ilmenau River and then shifted about 2 km further downstream. As water levels were observed at both places during the years 1951 and 1952, it was possible to adjust the whole time series for the present gauging site. Furthermore, summer water levels of the years 1900-1914 appeared slightly raised in relation to the values before and later. This was possibly due to backwater effects caused by an irrigation weir during this period or simply to inappropriate data management. Values of this time interval were decreased to fit into the time series before and after. This coarse correction diminishes the trend of the total time series. Its influence is thus "on the safe side". Fig. 3 shows the hydrographs of mean half-year water levels (summer: Mai-October, winter: November-April) of the whole time series. While short time variations reflect those of precipitation, significant negative trends are observed for the long term. Their correlation with the development of forest areas in the district (upper curve) is obvious. Interception of rainfall and water consumption by trees are reducing streamflows. As evapotranspiration is much higher in summer, the decrease of mean summer water levels is stronger than in winter. However, interception by pine trees remains and retention processes in the catchment during summer have also some impacts on winter conditions. The significant relationships between forest cover and water levels are shown in Fig. 4.

40 forest

16.5

30

correction

summer

16.0

20 water level

15.5 15.0 1885

10

winter

forest cover in %

w in m msl at Bienenbüttel

17.0

0 1905

1925

1945

1965

1985

Figure 3. Mean seasonal water levels at Bienenbüttel/Ilmenau River and forest cover of the catchment (District of Uelzen)

In the last decades the growth of forest areas slowed down with a higher proportion of deciduous woods. Since about 1960, the persisting negative trend of summer water levels is mainly caused by the abstraction of groundwater for sprinkler irrigation (Wittenberg, 2003). This becomes also obvious in Fig. 5, where mean half-year water levels are plotted against the corresponding values of forest cover. The dots marked "impact of groundwater abstraction" represent decreasing water levels without further increase of forest areas.

w m msl Bienenbüttel

16.5 summer R2 = 0.7 16.0

15.5 winter R2 = 0.43

impact of groundwater abstraction

15.0 20

25

30

35

forest area in % Figure 4.

Relationships between mean half-year water levels and forest cover of the catchment (R2 = squared correlation coefficient)

Low flows and baseflow as indicators for groundwater recharge and outflow are particularly affected by the decrease. This impact is demonstrated by low flow frequency analysis carried out for three time periods as shown in Fig. 5. The difference between the upper curve (1888-1917) and the middle one (1928-57) is essentially related to forestation. The further change shown by the lower curve (1965-94) is mainly due to groundwater abstractions for irrigation and drinking water needs.

15.8

NW m msl

15.6 15.4

1888-1917 1928-57 1965-94

15.2 15 14.8 0

10

20

return interval years Figure 5.

Low flow analysis, lowest water levels during the summer season (April – October) at the gauging station of Bienenbüttel/Ilmenau River

Water balance assessment

precipitation mm/a

It is part of human experience to search a tree for shelter during rainfall (interception) or to water plants (transpiration). The conclusion that forests intercept and consume water is nonetheless not really embedded in the awareness of people and decision makers. Discerned water balance changes in the district of Lunenburg were mostly attributed to agriculture. Schwind, 1949, identifies already a lowering of average water levels of the Ilmenau River during the period 1901-1945, i.e. long time before the beginning of groundwater abstractions for irrigation purposes in the fifties. He supposes however, that "increasing intensification of agriculture" and river training might have been the reasons. Forestry, though treated in his book, is not considered in this context. The influence of land use and forests on the water balance of catchments is principally known (Baumgartner, 1970) and has been subject of numerous studies (Sivapalan et al., 1996). Wessolek et al., 1985, determined the following water balance values for a typical low land area in Lower Saxony with sandy soils (near and similar to Lunenburg Heath): An average annual precipitation of 632 mm yielded an average groundwater recharge of 76 mm under coniferous forest against 221 mm with agricultural areas. Ernstberger, 1987, obtained evapotranspiration rates of 680 mm for spruce forest in the hill region of Hesse, Germany, and Müller, 1999 (in ATV-DVWK, 2002), found for pine forests ≥ 14 years with 620 mm annual precipitation in the North-German lowlands percolation heights between 0 und 83 mm. Wittenberg und Sivapalan, 1999, showed, that deep rooted vegetation does not only intercept infiltrating water but consumes groundwater, thus diminishing baseflow. In the present study the long time series of seasonal water levels were used to estimate changes of the water balance. For this purpose water levels had to be transformed into discharges. Approximate rating curves (winter and summer) were obtained by nonlinear regression of water levels and discharges available for the period 1955-94 and than applied to the whole series of recorded water levels to obtain respective discharges. Fig. 6 displays the corresponding runoff heights decreasing from about 260 mm in the year 1885 (20% of forest cover) to 187 mm today (36% forest). For compensating the flow reduction caused since 1955 by groundwater abstraction for irrigation, rising up to 33 mm in 1995 (Wittenberg, 2003), these values were added to actual runoff, yielding a total of 220 mm of runoff at that time. The time series of precipitation 1885 –1995 at Lunenburg have no significant trend; therefore a steady mean annual basin precipitation of 700 mm is taken into account. The given values allow establishing two water balance equations with two unknown terms, evapotranspiration from forests, ETforest, and from the other areas, ET.

700

700

600

600 evapotranspiration ET

500 400

500 400

ETforest

300

300

200

200 runoff

100 0 1888

ET ET forest abstraction runoff

100 0

1908

1928

1948

1968

1988

Figure 6. Decrease of mean annual runoff in the Ilmenau basin caused by forestation and groundwater abstraction (irrigation). Heights in mm refer to the total catchment area.

700 mm – 0.20 · ETforest - 0.80 · ET = 260 mm 700 mm – 0.36 · ETforest - 0.64 · ET = 220 mm

and

Solving this system yields ETforest = 640 mm and ET = 390 mm, thus runoff heights of 60 mm from forest areas and of 310 mm from agricultural and fallow lands. As data contain some uncertainty, sensitivity is tested: Assuming conservatively an annual runoff of only 250 mm in 1885, yields 100 mm runoff from forest areas and 288 mm from the rest of the basin. The historical scenarios elaborated by Bork et al., 1998, for the Elbe basin support the found magnitudes of effects of forestation and deforestation on the water balance. They estimated that runoff from the Elbe basin in the 7th century amounted only to 13% of precipitation as a 95% of the area was covered with forest. The decrease of forests due to wood exploitation, settling and cultivation with 20% of the watershed till the year 1310 increased runoff to 31% of precipitation causing a water mill boom at that time "Water mill theory". According to these findings, perennial streams in North Germany, the emergence and growth of bogs and wetlands in river plains, were mainly the result of human activities rather than elements of the natural landscape. In this sense, the decrease of groundwater recharge and runoff either caused by forestation or abstraction must not be necessarily considered as harmful or unnatural. Concerning irrigation practice, special care should rather be turned to the transport solids and substances and the quality of groundwater.

References ATV-DVWK,2002: Merkblatt M504, Verdunstung in Bezug zu Landnutzung, Bewuchs und Boden.(Evapotranspiration related to land use, vegetation and soil) Baumgartner, A., 1970: Water and energy balance of different vegetation covers. IAHS Proceedings of the Reading Symposium "World Water Balance. Bork, H.R., Bork, H., Dalchow, C., Faust, B., Piorr, H.P., Schatz, T., 1998:. Landschaftsentwicklung in Mitteleuropa.(Landscape development in Central Europe), Klett-Perthes, Gotha und Stuttgart. Ernstberger, H., 1987: Einfluss der Landnutzung auf Verdunstung und Wasserbilanz (Impact of land use on Evapotranspiration and water balance). Beiträge zur Hydrologie, Kirchzarten. Gils, H., 1962: Die wechselnde Abflusshemmung in verkrauteten Gewässern (Variation of . Deutsche Gewässerkdl. Mitt. 6, H. 5, 102-110. Leuschner, C., Immenroth, J., 1994: Landschaftsveränderungen in der Lüneburger Heide 17701985. Dokumentation und Bilanzierung auf der Grundlage historischer Karten(Landscape changes in the Lunenburg Heath 1770-1985. Arch. für Naturschutz u. Landschaftsforschung, 33, 85-139. Reimers, G., 1987: Schafe und Schäfer in der Lüneburger Heide (Sheep and shepherds in the Lunenburg Heath). Materialien Landwirtschaftsmuseum Lüneburger Heide Nr. 6. Sivapalan, M., Ruprecht, J., Viney, N., 1996: Water and SALT balance modelling to predict the effects of land-use changes in forested catchments. Hydrol. Process. 10, 391-411. Schwind, M., 1949. Der Landkreis Uelzen. Walter Dorn Verlag, Bremen. Wessolek, G., Renger, M., Facklam, M., Strebel, O., 1985: Einfluss von Standortnutzungsänderungen auf die Grundwasserneubildung (Effects of land use changes on groundwater recharge). Z. dt. geol. Ges. 136, 357-364. Wittenberg, H., 2003: Effects of season and man-made changes on baseflow and flow recession: case studies, Hydrological Processes, 17, 2113-2123. Wittenberg, H., Sivapalan, M., 1999: Watershed groundwater balance estimation using streamflow recession analysis and baseflow separation. Journal of Hydrology 219, 20-33.