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Quantification of local ecological effects in regional hydrologic modelling of bog reserves and surrounding agricultural lands
Agricultural water management ELSEVIER
AgriculturalWater Management25 (1994) 45-55
Quantification of local ecological effects in regional hydrologic modelling of bog reserves and surrounding agricultural lands P.E.V. van W a l s u m a'*, J.H.J. Joosten b aDLO Winand Staring Centre for Integrated Land, Soil and Water Research, P.O. Box 125, NL-6700 AC Wageningen, The Netherlands bLaboratory of Palaeobotany and Palynology, Heidelberglaan 2, NL-3584 CS Utrecht, The Netherlands
(Accepted 28 June 1993)
Abstract A bog reserve surrounded by agricultural lands can easily give rise to conflicts, especially if the subsoil has good water-transmitting properties. Hydrologic modelling can be used to quantify the linkage between water management in agricultural lands and the bog hydrology. However, most studies stop short of translating hydrologic effects into ecological ones. This is mainly due to the difference in scale between the hydrologic modelling and the fine-scaled landscape patterns in nature reserves. We developed a down-scaling method that attempts to bridge this gap. We assumed that for permanent bog growth to be possible, the long-term average of the downward seepage must not exceed a certain critical value. The area where the critical value is not exceeded was called the "Potential Bog Area". A method was devised for estimating changes in the size of this area. Such changes are also considered to be indicative for areal changes of other rare vegetation types in moist habitats. The method is illustrated using results from a study of the Groote Peel bog reserve in The Netherlands. Key words: Ecologicaleffect; Hydrologicmodelling;Bog reserve; Wetland;Drainage;Down-scaling
1. Introduction In a densely populated and intensively used country as The Netherlands, nature conservation necessarily began with the "left-overs" of socio-cconomic development. Only those *Correspondingauthor. 0378-3774/94/$07.00 © 1994ElsevierScienceB.V. All rights reserved S S D 1 0 3 7 8 - 3 7 7 4 ( 9 3 ) E0036-K
46
P.E.V. van Walsum, J.H.J. Joosten /Agricuhural WaterManagement25 (1994) 45-55
parts that could not be exploited any further were granted a conservation objective and were made into nature reserves. As a result, the location and configuration of these reserves is often unfavourable from a landscape-ecological point of view, leading to conflicts between neighbouring types of land use. These conflicts are especially pronounced in regions where the subsoil has good watertransmitting properties, This is usually the case in degenerated bog reserves in which former peat exploitation diminished the resistance to vertical flow. As a result, water management in the surrounding agricultural lands may have a significant impact on the water levels in the bog. An example is the installment of subsurface drains in wet pastures. The drains do not only lower the water level in the pastures but, inadvertently, also in the neighbouring bog. Several attempts have been made to quantify such impacts and to identify the extent to which hydrologic interventions in the surrounding area interfere with the bog hydrology. The subject has been reviewed by Poelman and Joosten (1992). None of the surveyed studies solve the problem of translating hydrologic effects into ecological ones. This is mainly due to the relatively coarse mesh of the hydrologic models in comparison to the finer-scaled landscape patterns of the bog reserves. In this paper we present a method to address this problem of scale incompatibility and describe its application to the Groote Peel bog reserve (The Netherlands).
2. The Groote Peel bog reserve
The Groote Peel bog reserve ( 1360 ha) is a remnant of the Peel bogs that extended over an area of 30 000 ha in the south-east of the Netherlands in centuries past. It contains a great variety in landscape types, including marshes and heathlands, stretches of purple moor grass (Molinia caerulea), brushwoods and forests, and has also a considerable extent of open water. Bog communities and plants are confined to peat pits where a local regeneration of bog growth has taken place (Joosten and Bakker, 1987). The reserve also contains a substantial area of moist heathlands, for which the Netherlands has an international conservation responsibility (Joosten and Van Noorden, 1992). The Groote Peel bog reserve was established in 1953 and has gradually expanded to its present size. Continued efforts have been made to raise the water level inside the bog. In 1980 the area was officially designated an internationally important wetland site by the Dutch Government according to the "Ramsar-Convention" of 1971. This convention is concerned with wetlands that serve as an important waterfowl habitat. In 1985 the procedure to make the Groote Peel into a national park was initiated and in 1986 the reserve was designated a "special protection area" under the EC Birds Directive (Vermeer and Joosten, 1992). Reacting against these developments, the farmers in the surrounding area began to install subsurface drainage. As a result, about 38% of the wetter soils surrounding the bog reserve have actually been drained since 1985 (Landinrichtingsdienst, 1988). Then, in 1989 the Dutch Government designated a buffer zone, forbidding further hydrologic interventions within 600 m from the bog reserve. This zone was subsequently extended to 2000 m in 1990 (Vermeer and Joosten, 1992). Currently, negotiations are taking place about supplying surface water to the agricultural area surrounding the Groote Peel, intended for infiltration
P.E. V. van Walsum, J.H.J. Joosten /Agricultural Water Management 25 (1994) 45-55
47
in the ditches that normally fall dry during the summer period. This will raise the hydraulic head in the subsoil, thereby reducing the downward seepage from the bog reserve.
3. S i m u l a t i o n o f t h e r e g i o n a l h y d r o l o g y
Crop production in the Netherlands can become limited either due to an excess or to a deficit of soil moisture. The measures taken to mitigate the influence of varying moisture conditions are: - the installment of subsurface drains; - the installment of (mobile) sprinklers. Both drains and sprinklers function intermittently as a result of changing hydrologic conditions in the shallow subsoil. Thus, their impacts can only be adequately modelled with a transient model that includes the unsaturated zone, such as SIMGRO (Querner, 1988). SIMGRO models a regional hydrologic system in an integrated manner, covering groundwater, surface water, and soil moisture. The model mainly focuses on the (saturated) groundwater system and the integrating role it plays on the regional scale. SIMGRO uses two levels of spatial discretization: - nodes of the finite-element scheme for simulating the groundwater flow;
- homogeneous "subregions" for modelling the unsaturated zone and the surface water system; each subregion comprises a number of nodes (usually 5-20). SIMGRO was set up for the Groote Peel and surrounding area (total of 10000 ha), with a geohydrologic representation involving four layers. The top ("phreatic") layer has the characteristic of an aquitard, owing to the presence of loam in the shallow subsoil. Underneath the system is an impermeable layer at a depth of about 300 m. For the spatial discretization 456 nodes and 83 subregions have been used; 16 subregions are within the Groote Peel. Groundwater level measurements of 1985-1986 were used for calibrating the drainage resistances (Poelman, 1987). Discharge measurements of 1987-1988 were used for calibrating the vertical flow resistance of the top geohydrologic layer inside the bog reserve. This model parameter plays a key role in simulating the interaction between the bog reserve and the surrounding area. In Table 1 a comparison is made between simulated and measured discharges for the calibration period, for a number of subregions within the Groote Peel. Validation of the model using groundwater level and discharge data of 1986-1987 showed a very reasonable agreement between model predictions and measurements. For this period the measured discharge from the Groote Peel totalled 1.5 million m 3, and the simulated discharge 1.1 million m 3. This difference is acceptable if one considers that the discharge is a component of an annual water balance totalling 9 million m 3 (Van Walsum, 1992). For scenario-simulation purposes, the model was run with meteorologic data of the period 1971-1981. The computed long-term average of the downward seepage for the bog reserve as a whole was found to be 160 m m . y r - i . This long-term average can only be computed
48
P.E. V, van Walsum, J.H.J, Joosten /Agricultural Water Management 25 (1994) 45-55
Table 1 Comparison between measured and simulated discharges for a number of subregions within the Groote Peel for the calibration period November 1 1988 to May 16 1989 Subregion
Area (ha)
Measured discharge ( 103 m 3)
Simulated discharge ( 103 m 3)
Difference (%)
2 9 16
99 103 197
110 56 323
114 50 353
+4 -11 +9
with a transient model because the system contains many forms of non-linearity (e.g. thresholds in the interaction between groundwater and surface water). Thus the "average effect of varying meteorological conditions" is not equal to the "effect of average meteorological conditions". As can be seen from Fig. I a there is a considerable variation between the subregions; also the average water level varies considerably. Results are shown at the resolution of subregions because the model was calibrated at this level. Simulations with the model showed that unrestricted subsurface drainage in the surrounding area would increase the downward seepage from the bog reserve by about 10 m m . y r - J, which is a 6% increase of the current long-term average value (Van Walsum, 1990). On an annual basis the increase varies between 3 and 9%. Unrestricted expansion of sprinker irrigation would add a further 3% to the downward seepage, because of extra groundwater a
b ,al
!
0
1
i
x
2kin .
0 t
i
Downward seepage (rnm.yr "1)
1 J
2kin
Water level below soil surface (cm)
t11- iso
~
171-190
3 0 40
~
6 1 75
131-150
~
191-220
,1 5 0
~
7690
151 - 170
51 -60
Fig. 1. Subregional averages of the long-term average of the downward seepage within the Groote Peel (a) and the average water level (b) as simulated using the meteorologic data for the period 1971-1981.
P.E. V. van Walsum, J.H.J. Joosten /Agricultural Water Management 25 (1994)45-55
49
extraction. On the other hand, implementation of water supply schemes in the surrounding agricultural area could reduce the downward seepage by about 15%. The costs of these schemes would largely be covered by the extra income from the increased crop production, as a result of the increase in capillary rise from groundwater. Further reduction of the downward seepage is possible if even more radical measures are taken in the surrounding area, such as creating a zone of swamped land around the bog reserve.
4. Ecological
effects
Ecological impacts of measures in the surrounding agricultural lands on the Groote Peel were described in a qualitative way by DHV (1989) and Project Groep de Groote Peel (1990). However, more quantitative ecological data are required for the purpose of decision making.
4.1. Hydrologic conditionsfor bog growth With respect to the conservation objectives of the Groote Peel, the key species are bog mosses of the genus Sphagnum. These mosses are highly sensitive to both the quality and quantity of water. They can only thrive in an ombrotrophic environment where only precipitation water reaches the substrate. The water level should roughly remain within 20 cm of the soil surface. These requirements can only be fulfilled if a number of conditions are met: the drainage base must be shallow. This means that surface water outflow is blocked, except in very wet periods; - the local storage capacity (porosity and the presence of open water) must be very high; - the seepage must be directed downwards and must not exceed a certain limit. -
If one or more of these conditions is not satisfied, the growth of the mosses is inhibited, though a certain amount of mutual "substitution" is possible. The downward seepage is the hydrologic "interface" variable between external hydrologic changes and internal ecological effects. From practical observations in other bog reserves we estimated that for the prevailing climatologic conditions, the long-term average of the downward seepage should not be more than about 30-50 mm.yr-1. This is only an approximate value that should not be interpreted as a strict threshold. An ecosystem may follow a dynamic pattern of expansion during" good" years and contraction during " b a d " years. Even several "bad" years in a row do not necessarily mean that the system will not survive. Furthermore, the presence of bog species is not only determined by their physiological constraints, but also by competition with other species. In the case of bog communities, the "drier" species may be eliminated by periodic wetter conditions. For these reasons the long-term average of the downward seepage can be considered as a good parameter for determining the long-term survival chances of bog mosses. The computed downward seepage in the Groote Peel (Fig. la) was everywhere much higher than the assumed critical value (30-50 mm. y r - ~). This could suggest that in the
P.E. V. van Walsum, J.H.J. Joosten /Agricultural Water Management 25 (1994) 45-55
50
current conditions bog-forming vegetation is not sustainable and that external measures do not affect the ecological value of the reserve. However, this line of reasoning can not be upheld: there is a continuous presence of living bog vegetation, even though it is limited to a very small area (less than 1 ha in the whole reserve). The paradox can be explained by spatial variations in the peat thickness. Since (humified) peat has a particularly high flow resistance, the downward seepage is locally very low at the places where the peat is relatively thick. Furthermore, apart from some patches with living bog communities, the Groote Peel contains other communities that are hydrologically less demanding, but nonetheless important from a nature conservation point of view. Though the average thickness ofhumified peat is not more than 30 cm, the actual thickness can vary between zero and a few meters (Joosten and Bakker, 1987). These two extremes correspond to two cases in which the potential success of the bog-forming vegetation is not influenced by external conditions: at places with no humified peat at all the vertical flow resistance may be so small and the downward seepage so high that re-establishment of the bog-forming vegetation will not be possible; - at places with a very thick layer of humified peat the local hydrology is virtually insulated from the regional system, leading to a negligible influence of external measures. -
Between these two extremes, there must be a transition zone which is sensitive to external measures. In this zone the downward seepage is close to the critical (maximum allowable) value and the peat has a corresponding minimum thickness. This thickness will be different for different values of the hydraulic head in the underlying aquifer. A decrease in the hydraulic head will cause an increase in the average downward seepage. This in turn will necessitate a larger thickness to compensate for the increased downward force on the water. The bigger the necessary thickness, the smaller the percentage of the area where the downward seepage is equal to or less than the critical value.
4.2. Quantitative method for evaluating ecological impact This line of argument has been developed into a quantitative method for down-scaling results of hydrologic modelling to the site characteristics of bog mosses. The vertical resistance of the top layer of the four-layer geohydrologic schematization is only partly due to the presence of peat. The remainder is caused by the presence of loamy material in the subsoil directly beneath the peat. The vertical resistance of the latter can be considered to be constant within a subregion, because the thickness of the loam layers does not vary much (Joosten and Bakker, 1987). When the peat thickness has been mapped in terms of "classes", as has been done by Joosten and Bakker (1987), the local resistance can be calculated as:
C.,, = C.b ÷ C.~d,, where
Ci4,
=
(l)
total vertical flow resistance of the top layer for a peat thickness class p in
P.E.V. van Walsum, J.H.J. Joosten / Agricultural Water Management 25 (1994) 45-55
51
subregion i (d); Ci,b= vertical flow resistance of the loamy subsoil (d) ; C~,~= specific vertical flow resistance of the peat (d. m - 1); dp = average peat thickness of class p (m) A uniform drop of the hydraulic head was assumed between the top layer and the first aquifer within a subregion of the Groote Peel. This drop can be computed in an approximate fashion as:
Ahi = WiCi
(2)
where Ahi= hydraulic head drop (long-term average) between the top layer and the first aquifer, in subregion i (m); Wi = long-term average of the downward seepage ( m . d - 1) and Ci = average vertical flow resistance of the top layer (d). For each peat thickness class the long-term average of the downward seepage, W~,p,can be expressed as:
Wi,p = Ahi/ Ci,p
(3)
The sum of the downward seepages (of the separate classes) multiplied by their respective areal fractions, AFi,p, m u s t then equal the subregional long-term average:
Wi = ~pAFi,p W~,p
(4)
If Ci.b is the assumed value of the resistance of the shallow loam layer then the only unknown in the above equations is the specific resistance Ci,~;. This parameter can be calculated by substituting Eqs. (1) and (2) in Eq. (3) and subsequently Eq. (3) in Eq. (4). The downward seepage (calculated with Eq. 3) of each peat thickness class can be graphically represented in the form of a block diagram (Fig. 2a). The total area of the blocks is equal to the subregional downward seepage (Eq. 4). It is possible to draw a curved line through the centres of the blocks, thus approximating the continuous form of the relationship between the cumulative areal fraction and the downward seepage (Fig. 2b). Apart from the curve for the current situation, the curves for various changes of the average seepage are also drawn in ( + 15%, + 30%, - 15%, - 30%). Next, the intersections with an assumed critical level of the downward seepage can be determined. In Fig. 2c this has been done for both levels of 30 and 50 m m . y r - 1 . The fraction of the area (of a subregion) with a downward seepage less than the critical value is equal to the complementary fraction ( F / ) .
4.3. Method application The procedure described above was applied to all of the 16 subregions used for modelling the Groote Peel. The calibrated values of Ci vary between 650 and 1900 d. An arbitrary value of 2/3 Ci was used for the value of Ci,b in Eq. ( 1 ), because of the lack of adequate data. This means that only one-third of the resistance was attributed to the presence of peat, leading to an underestimation of the variability of the vertical resistance and hence of the area calculated to have less than the critical seepage amount. The calculated values of the specific resistance Ci,,~vary between 1400 and 8400 d. m 1, which corresponds to saturated
P.E. V. van Walsum, J.H.J. ]oosten / Agricultural Water Management 25 (1994)45-55
52
a
300
"7,
250
E E 200
150
100 D
50 I
I
I
I
I
30% .~.
300
i
I
b
15% ~
"~'""~.
0% ~
'~
250 E
200
-15%
~
"'~. ~'-.~
"~..~
~" 150 7o 100 c
...... : L T Z ~ . . . .
~ o
c~
50
I
I
I
l
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I
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Li l ~" ~'~"
i,
F 30% - . . . .
300
15% ~
C "'~.
250 E E 200
~" 150
.~ ~00 c o ~
£3
5O 30 0.0
I
I
I
0.10
0.20
0.30
I
I
I
0.40 0.50 0.60 Cumulative areal fraction ( - - )
I
I
I
0.70
0.60
o.go
i "F
1.00 c
"
Fig. 2. Steps in determining (per subregion) the change of area where the downward seepage is less than a certain critical value: (a) downward seepage per peat thickness class (1-6). (b) Seepage as a function of the cumulative fraction for the current situation and with increased ( + 15% and + 30%) and decreased ( - 15% and - 3 0 % ) seepage. The downward seepage of a given fraction F is equal to or greater than WF. (C) The complementary fraction F ' for two different assumed levels of the critical downward seepage (30 and 50 mm. y r - ~).
P.E. V. van Walsum, J.H.J. Joosten /Agricultural Water Management 25 (1994) 45-55
53
conductivities of 0.71 and 0.12 mm- d - 1. This variation is the result of local differences in the type of peat and the degree of humification. For a critical downward seepage of 30 mm. yr 1, the calculated area with a less than critical seepage was about 38 ha (3% of the G r o o t e P e e l area); for a critical downward seepage of 50 m m . y r - 1 this area was about 88 ha (7% of the G r o o t e P e e l area). These values are similar to the actual area of rare groundwater-dependent vegetation types in the G r o o t e P e e l (Van Walsum, 1992). However, these areas only give an indication of the area that complies with a certain seepage criterion. Apart from a maximum level of downward seepage, the other hydrologic conditions for bog growth also have to be fulfilled (i.e. the shallow drainage base and the high local storage capacity of the substrate). Because these conditions will not be found everywhere, a certain reduction of the mentioned areas should be applied. This problem was circumvented by presenting all results as percentage changes of the area where the seepage criterion is fulfilled - the latter we called the "Potential Bog Area". The relative change appeared to be roughly the same for both 30 and 50 mm. yr 1 of downward seepage; thus the method does not require knowing the exact value of the critical downward seepage. In a number of scenarios (Van Walsum, 1990) the computed change of the "Potential Bog Area" has been plotted against the change of the downward seepage (of the G r o o t e P e e l as a whole). Fig. 3 shows a regression curve through the points. Less downward seepage, resulting in more stable water levels is also favourable for vegetation types that
APBA (%) 80,
75,
60,
45.
30 2
15'
O'
-15. I I I
-30 -25
I
I
I
I
I
I
I
-20
-15
-10
-5
0
5
10
AW (%)
Fig. 3. Approximaterelationship between the changeof the long-termaverage seepagefromthe Groote Peel as a whole (AW) and the "Potential Bog Area" (APBA).
54
P.E. V. van Walsum, J.H.J. Joosten /Agricultural Water Management 25 (1994) 45-55
are less demanding than living bog. Therefore, a reduction of downward seepage will lead to a general ecological upgrading of the area. For this reason the "Potential Bog A r e a " was used as an overall ecological standard for the G r o o t e Peel.
5. Concluding remarks A method has been presented for quantifying the local ecological impacts of regional hydrologic interventions on a degenerated bog. It was needed to bridge the gap between the spatial resolution with which the regional hydrologic model could be calibrated (subregions with an average area of 80 ha) and the resolution required for predicting the effect on the bog vegetation (a few hundred square metres, or even only a few square metres). In the future it will be possible to do regional hydrologic modelling with a very fine mesh, However, this is not only a matter of having enough computing power. More importantly, detailed hydrologic information about the situation in the field is necessary. Especially in landscapes with a large fine-scaled diversity, i.e. in bogs that have been subject to exploitation in the past, obtaining all these data in a non-destructive manner is an impossible task. For this reason, it will always be necessary to devise an approximate procedure for downscaling results of regional hydrologic modelling to local ecological impacts. Such a procedure will often only predict the (changing) area of valuable vegetation, without being able to pinpoint the exact spots where the changes will take place. Though the straightforward applicability to other types of hydrologic-ecological interactions may seem limited, the present case study does illustrate the kind of approach that could be followed. In the case of the G r o o t e P e e l this approach has played an important role in the decision-making process leading up to the designation of a 2000 m wide buffer zone (Vermeer and Joosten, 1992).
6. References DHV, 1989. Ecologische effecten van drainage op de Groote Peel. DHV Raadgevend Ingenieursbureau,Amersfoort (The Netherlands), 64 pp. (in Dutch) Joosten, J.H.J. and Bakker, T.W.M., 1987. De Groote Peel in verleden, heden en toekomst. Staatsbosbeheer, Utrecht (The Netherlands). Rapport 88-4. 291 pp. (in Dutch) Joosten, J.H.J. and Van Noorden, B.P.M., 1992. De Groote Peel: leren waarderen; Een oefening in het waarderen van natuurelemententen behoeve van het natuurbehoud. NatuurhistorischMaandblad,81 : 203-211. ( in Dutch) Landinrichtingsdienst, 1988. De waterhuishouding van de Groote Peel. Landinrichtingsdienst, Utrecht (The Netherlands), 24 pp. (in Dutch) Poelman, A., 1987. Geohydrologische modelstudie van de Groote Peel en omgeving. Staatsbosbeheer, Utrecht (The Netherlands). Rapport 88-3, 60 pp. ( in Dutch) Poelman, A. and Joosten, J.H.J., 1992. On the identification of hydrological interaction zones for bog reserves. In: O.M. Bragg, P.D. Hulme, H.A.P. Ingram and R.A. Robertson (Editors), Peatland Ecosystems and Man: An hnpact Assessment. Dept. of Biological Sciences University of Dundee, Dundee, pp. 141-148. Projectgroep de Groote Peel, 1990. Technische maatregelen ter verbetering van de waterhuishoudingin de Groote Peel en hun effecten. Ministerie van Landbouw, Natuurbeheer en Visserij, Den Haag (The Netherlands), 63 pp. (in Dutch)
P.E. V. van Walsum, J.H.Z Joosten / Agricultural Water Management 25 (1994) 45-55
55
Querner, E.P., 1988. Description of a regional groundwater flow model SIMGRO and some applications. Agric. Water Manage., 14: 209-218. Van Walsum, P.E.V., 1990. Waterbeheer rondom de Groote Peel; verkenning en evaluatie van scenario's. DLO Staring Centrum, Wageningen (The Netherlands). Rapport 106, 106 pp. (in Dutch) Van Walsum, P.E.V., 1992. Water management in the Groote Peel bog reserve and surrounding agricultural area; simulation and optimization. DLO Winand Staring Centre, Wageningen (The Netherlands). Report 49, 124 pp. Vermeer, J.G. and Joosten, J.H.J., 1992. Conservation and management of bog and fen reserves in the Netherlands. In: J.T.A. Verhoeven (Editor), Fens and Bogs in The Netherlands: Vegetation, History, Nutrient Dynamics and Conservation. Geobotany, 18. Kluwer Academic Publishers, Dordrecht, pp. 433-478.
AgriculturalWater Management25 (1994) 45-55
Quantification of local ecological effects in regional hydrologic modelling of bog reserves and surrounding agricultural lands P.E.V. van W a l s u m a'*, J.H.J. Joosten b aDLO Winand Staring Centre for Integrated Land, Soil and Water Research, P.O. Box 125, NL-6700 AC Wageningen, The Netherlands bLaboratory of Palaeobotany and Palynology, Heidelberglaan 2, NL-3584 CS Utrecht, The Netherlands
(Accepted 28 June 1993)
Abstract A bog reserve surrounded by agricultural lands can easily give rise to conflicts, especially if the subsoil has good water-transmitting properties. Hydrologic modelling can be used to quantify the linkage between water management in agricultural lands and the bog hydrology. However, most studies stop short of translating hydrologic effects into ecological ones. This is mainly due to the difference in scale between the hydrologic modelling and the fine-scaled landscape patterns in nature reserves. We developed a down-scaling method that attempts to bridge this gap. We assumed that for permanent bog growth to be possible, the long-term average of the downward seepage must not exceed a certain critical value. The area where the critical value is not exceeded was called the "Potential Bog Area". A method was devised for estimating changes in the size of this area. Such changes are also considered to be indicative for areal changes of other rare vegetation types in moist habitats. The method is illustrated using results from a study of the Groote Peel bog reserve in The Netherlands. Key words: Ecologicaleffect; Hydrologicmodelling;Bog reserve; Wetland;Drainage;Down-scaling
1. Introduction In a densely populated and intensively used country as The Netherlands, nature conservation necessarily began with the "left-overs" of socio-cconomic development. Only those *Correspondingauthor. 0378-3774/94/$07.00 © 1994ElsevierScienceB.V. All rights reserved S S D 1 0 3 7 8 - 3 7 7 4 ( 9 3 ) E0036-K
46
P.E.V. van Walsum, J.H.J. Joosten /Agricuhural WaterManagement25 (1994) 45-55
parts that could not be exploited any further were granted a conservation objective and were made into nature reserves. As a result, the location and configuration of these reserves is often unfavourable from a landscape-ecological point of view, leading to conflicts between neighbouring types of land use. These conflicts are especially pronounced in regions where the subsoil has good watertransmitting properties, This is usually the case in degenerated bog reserves in which former peat exploitation diminished the resistance to vertical flow. As a result, water management in the surrounding agricultural lands may have a significant impact on the water levels in the bog. An example is the installment of subsurface drains in wet pastures. The drains do not only lower the water level in the pastures but, inadvertently, also in the neighbouring bog. Several attempts have been made to quantify such impacts and to identify the extent to which hydrologic interventions in the surrounding area interfere with the bog hydrology. The subject has been reviewed by Poelman and Joosten (1992). None of the surveyed studies solve the problem of translating hydrologic effects into ecological ones. This is mainly due to the relatively coarse mesh of the hydrologic models in comparison to the finer-scaled landscape patterns of the bog reserves. In this paper we present a method to address this problem of scale incompatibility and describe its application to the Groote Peel bog reserve (The Netherlands).
2. The Groote Peel bog reserve
The Groote Peel bog reserve ( 1360 ha) is a remnant of the Peel bogs that extended over an area of 30 000 ha in the south-east of the Netherlands in centuries past. It contains a great variety in landscape types, including marshes and heathlands, stretches of purple moor grass (Molinia caerulea), brushwoods and forests, and has also a considerable extent of open water. Bog communities and plants are confined to peat pits where a local regeneration of bog growth has taken place (Joosten and Bakker, 1987). The reserve also contains a substantial area of moist heathlands, for which the Netherlands has an international conservation responsibility (Joosten and Van Noorden, 1992). The Groote Peel bog reserve was established in 1953 and has gradually expanded to its present size. Continued efforts have been made to raise the water level inside the bog. In 1980 the area was officially designated an internationally important wetland site by the Dutch Government according to the "Ramsar-Convention" of 1971. This convention is concerned with wetlands that serve as an important waterfowl habitat. In 1985 the procedure to make the Groote Peel into a national park was initiated and in 1986 the reserve was designated a "special protection area" under the EC Birds Directive (Vermeer and Joosten, 1992). Reacting against these developments, the farmers in the surrounding area began to install subsurface drainage. As a result, about 38% of the wetter soils surrounding the bog reserve have actually been drained since 1985 (Landinrichtingsdienst, 1988). Then, in 1989 the Dutch Government designated a buffer zone, forbidding further hydrologic interventions within 600 m from the bog reserve. This zone was subsequently extended to 2000 m in 1990 (Vermeer and Joosten, 1992). Currently, negotiations are taking place about supplying surface water to the agricultural area surrounding the Groote Peel, intended for infiltration
P.E. V. van Walsum, J.H.J. Joosten /Agricultural Water Management 25 (1994) 45-55
47
in the ditches that normally fall dry during the summer period. This will raise the hydraulic head in the subsoil, thereby reducing the downward seepage from the bog reserve.
3. S i m u l a t i o n o f t h e r e g i o n a l h y d r o l o g y
Crop production in the Netherlands can become limited either due to an excess or to a deficit of soil moisture. The measures taken to mitigate the influence of varying moisture conditions are: - the installment of subsurface drains; - the installment of (mobile) sprinklers. Both drains and sprinklers function intermittently as a result of changing hydrologic conditions in the shallow subsoil. Thus, their impacts can only be adequately modelled with a transient model that includes the unsaturated zone, such as SIMGRO (Querner, 1988). SIMGRO models a regional hydrologic system in an integrated manner, covering groundwater, surface water, and soil moisture. The model mainly focuses on the (saturated) groundwater system and the integrating role it plays on the regional scale. SIMGRO uses two levels of spatial discretization: - nodes of the finite-element scheme for simulating the groundwater flow;
- homogeneous "subregions" for modelling the unsaturated zone and the surface water system; each subregion comprises a number of nodes (usually 5-20). SIMGRO was set up for the Groote Peel and surrounding area (total of 10000 ha), with a geohydrologic representation involving four layers. The top ("phreatic") layer has the characteristic of an aquitard, owing to the presence of loam in the shallow subsoil. Underneath the system is an impermeable layer at a depth of about 300 m. For the spatial discretization 456 nodes and 83 subregions have been used; 16 subregions are within the Groote Peel. Groundwater level measurements of 1985-1986 were used for calibrating the drainage resistances (Poelman, 1987). Discharge measurements of 1987-1988 were used for calibrating the vertical flow resistance of the top geohydrologic layer inside the bog reserve. This model parameter plays a key role in simulating the interaction between the bog reserve and the surrounding area. In Table 1 a comparison is made between simulated and measured discharges for the calibration period, for a number of subregions within the Groote Peel. Validation of the model using groundwater level and discharge data of 1986-1987 showed a very reasonable agreement between model predictions and measurements. For this period the measured discharge from the Groote Peel totalled 1.5 million m 3, and the simulated discharge 1.1 million m 3. This difference is acceptable if one considers that the discharge is a component of an annual water balance totalling 9 million m 3 (Van Walsum, 1992). For scenario-simulation purposes, the model was run with meteorologic data of the period 1971-1981. The computed long-term average of the downward seepage for the bog reserve as a whole was found to be 160 m m . y r - i . This long-term average can only be computed
48
P.E. V, van Walsum, J.H.J, Joosten /Agricultural Water Management 25 (1994) 45-55
Table 1 Comparison between measured and simulated discharges for a number of subregions within the Groote Peel for the calibration period November 1 1988 to May 16 1989 Subregion
Area (ha)
Measured discharge ( 103 m 3)
Simulated discharge ( 103 m 3)
Difference (%)
2 9 16
99 103 197
110 56 323
114 50 353
+4 -11 +9
with a transient model because the system contains many forms of non-linearity (e.g. thresholds in the interaction between groundwater and surface water). Thus the "average effect of varying meteorological conditions" is not equal to the "effect of average meteorological conditions". As can be seen from Fig. I a there is a considerable variation between the subregions; also the average water level varies considerably. Results are shown at the resolution of subregions because the model was calibrated at this level. Simulations with the model showed that unrestricted subsurface drainage in the surrounding area would increase the downward seepage from the bog reserve by about 10 m m . y r - J, which is a 6% increase of the current long-term average value (Van Walsum, 1990). On an annual basis the increase varies between 3 and 9%. Unrestricted expansion of sprinker irrigation would add a further 3% to the downward seepage, because of extra groundwater a
b ,al
!
0
1
i
x
2kin .
0 t
i
Downward seepage (rnm.yr "1)
1 J
2kin
Water level below soil surface (cm)
t11- iso
~
171-190
3 0 40
~
6 1 75
131-150
~
191-220
,1 5 0
~
7690
151 - 170
51 -60
Fig. 1. Subregional averages of the long-term average of the downward seepage within the Groote Peel (a) and the average water level (b) as simulated using the meteorologic data for the period 1971-1981.
P.E. V. van Walsum, J.H.J. Joosten /Agricultural Water Management 25 (1994)45-55
49
extraction. On the other hand, implementation of water supply schemes in the surrounding agricultural area could reduce the downward seepage by about 15%. The costs of these schemes would largely be covered by the extra income from the increased crop production, as a result of the increase in capillary rise from groundwater. Further reduction of the downward seepage is possible if even more radical measures are taken in the surrounding area, such as creating a zone of swamped land around the bog reserve.
4. Ecological
effects
Ecological impacts of measures in the surrounding agricultural lands on the Groote Peel were described in a qualitative way by DHV (1989) and Project Groep de Groote Peel (1990). However, more quantitative ecological data are required for the purpose of decision making.
4.1. Hydrologic conditionsfor bog growth With respect to the conservation objectives of the Groote Peel, the key species are bog mosses of the genus Sphagnum. These mosses are highly sensitive to both the quality and quantity of water. They can only thrive in an ombrotrophic environment where only precipitation water reaches the substrate. The water level should roughly remain within 20 cm of the soil surface. These requirements can only be fulfilled if a number of conditions are met: the drainage base must be shallow. This means that surface water outflow is blocked, except in very wet periods; - the local storage capacity (porosity and the presence of open water) must be very high; - the seepage must be directed downwards and must not exceed a certain limit. -
If one or more of these conditions is not satisfied, the growth of the mosses is inhibited, though a certain amount of mutual "substitution" is possible. The downward seepage is the hydrologic "interface" variable between external hydrologic changes and internal ecological effects. From practical observations in other bog reserves we estimated that for the prevailing climatologic conditions, the long-term average of the downward seepage should not be more than about 30-50 mm.yr-1. This is only an approximate value that should not be interpreted as a strict threshold. An ecosystem may follow a dynamic pattern of expansion during" good" years and contraction during " b a d " years. Even several "bad" years in a row do not necessarily mean that the system will not survive. Furthermore, the presence of bog species is not only determined by their physiological constraints, but also by competition with other species. In the case of bog communities, the "drier" species may be eliminated by periodic wetter conditions. For these reasons the long-term average of the downward seepage can be considered as a good parameter for determining the long-term survival chances of bog mosses. The computed downward seepage in the Groote Peel (Fig. la) was everywhere much higher than the assumed critical value (30-50 mm. y r - ~). This could suggest that in the
P.E. V. van Walsum, J.H.J. Joosten /Agricultural Water Management 25 (1994) 45-55
50
current conditions bog-forming vegetation is not sustainable and that external measures do not affect the ecological value of the reserve. However, this line of reasoning can not be upheld: there is a continuous presence of living bog vegetation, even though it is limited to a very small area (less than 1 ha in the whole reserve). The paradox can be explained by spatial variations in the peat thickness. Since (humified) peat has a particularly high flow resistance, the downward seepage is locally very low at the places where the peat is relatively thick. Furthermore, apart from some patches with living bog communities, the Groote Peel contains other communities that are hydrologically less demanding, but nonetheless important from a nature conservation point of view. Though the average thickness ofhumified peat is not more than 30 cm, the actual thickness can vary between zero and a few meters (Joosten and Bakker, 1987). These two extremes correspond to two cases in which the potential success of the bog-forming vegetation is not influenced by external conditions: at places with no humified peat at all the vertical flow resistance may be so small and the downward seepage so high that re-establishment of the bog-forming vegetation will not be possible; - at places with a very thick layer of humified peat the local hydrology is virtually insulated from the regional system, leading to a negligible influence of external measures. -
Between these two extremes, there must be a transition zone which is sensitive to external measures. In this zone the downward seepage is close to the critical (maximum allowable) value and the peat has a corresponding minimum thickness. This thickness will be different for different values of the hydraulic head in the underlying aquifer. A decrease in the hydraulic head will cause an increase in the average downward seepage. This in turn will necessitate a larger thickness to compensate for the increased downward force on the water. The bigger the necessary thickness, the smaller the percentage of the area where the downward seepage is equal to or less than the critical value.
4.2. Quantitative method for evaluating ecological impact This line of argument has been developed into a quantitative method for down-scaling results of hydrologic modelling to the site characteristics of bog mosses. The vertical resistance of the top layer of the four-layer geohydrologic schematization is only partly due to the presence of peat. The remainder is caused by the presence of loamy material in the subsoil directly beneath the peat. The vertical resistance of the latter can be considered to be constant within a subregion, because the thickness of the loam layers does not vary much (Joosten and Bakker, 1987). When the peat thickness has been mapped in terms of "classes", as has been done by Joosten and Bakker (1987), the local resistance can be calculated as:
C.,, = C.b ÷ C.~d,, where
Ci4,
=
(l)
total vertical flow resistance of the top layer for a peat thickness class p in
P.E.V. van Walsum, J.H.J. Joosten / Agricultural Water Management 25 (1994) 45-55
51
subregion i (d); Ci,b= vertical flow resistance of the loamy subsoil (d) ; C~,~= specific vertical flow resistance of the peat (d. m - 1); dp = average peat thickness of class p (m) A uniform drop of the hydraulic head was assumed between the top layer and the first aquifer within a subregion of the Groote Peel. This drop can be computed in an approximate fashion as:
Ahi = WiCi
(2)
where Ahi= hydraulic head drop (long-term average) between the top layer and the first aquifer, in subregion i (m); Wi = long-term average of the downward seepage ( m . d - 1) and Ci = average vertical flow resistance of the top layer (d). For each peat thickness class the long-term average of the downward seepage, W~,p,can be expressed as:
Wi,p = Ahi/ Ci,p
(3)
The sum of the downward seepages (of the separate classes) multiplied by their respective areal fractions, AFi,p, m u s t then equal the subregional long-term average:
Wi = ~pAFi,p W~,p
(4)
If Ci.b is the assumed value of the resistance of the shallow loam layer then the only unknown in the above equations is the specific resistance Ci,~;. This parameter can be calculated by substituting Eqs. (1) and (2) in Eq. (3) and subsequently Eq. (3) in Eq. (4). The downward seepage (calculated with Eq. 3) of each peat thickness class can be graphically represented in the form of a block diagram (Fig. 2a). The total area of the blocks is equal to the subregional downward seepage (Eq. 4). It is possible to draw a curved line through the centres of the blocks, thus approximating the continuous form of the relationship between the cumulative areal fraction and the downward seepage (Fig. 2b). Apart from the curve for the current situation, the curves for various changes of the average seepage are also drawn in ( + 15%, + 30%, - 15%, - 30%). Next, the intersections with an assumed critical level of the downward seepage can be determined. In Fig. 2c this has been done for both levels of 30 and 50 m m . y r - 1 . The fraction of the area (of a subregion) with a downward seepage less than the critical value is equal to the complementary fraction ( F / ) .
4.3. Method application The procedure described above was applied to all of the 16 subregions used for modelling the Groote Peel. The calibrated values of Ci vary between 650 and 1900 d. An arbitrary value of 2/3 Ci was used for the value of Ci,b in Eq. ( 1 ), because of the lack of adequate data. This means that only one-third of the resistance was attributed to the presence of peat, leading to an underestimation of the variability of the vertical resistance and hence of the area calculated to have less than the critical seepage amount. The calculated values of the specific resistance Ci,,~vary between 1400 and 8400 d. m 1, which corresponds to saturated
P.E. V. van Walsum, J.H.J. ]oosten / Agricultural Water Management 25 (1994)45-55
52
a
300
"7,
250
E E 200
150
100 D
50 I
I
I
I
I
30% .~.
300
i
I
b
15% ~
"~'""~.
0% ~
'~
250 E
200
-15%
~
"'~. ~'-.~
"~..~
~" 150 7o 100 c
...... : L T Z ~ . . . .
~ o
c~
50
I
I
I
l
I
I
I
l
Li l ~" ~'~"
i,
F 30% - . . . .
300
15% ~
C "'~.
250 E E 200
~" 150
.~ ~00 c o ~
£3
5O 30 0.0
I
I
I
0.10
0.20
0.30
I
I
I
0.40 0.50 0.60 Cumulative areal fraction ( - - )
I
I
I
0.70
0.60
o.go
i "F
1.00 c
"
Fig. 2. Steps in determining (per subregion) the change of area where the downward seepage is less than a certain critical value: (a) downward seepage per peat thickness class (1-6). (b) Seepage as a function of the cumulative fraction for the current situation and with increased ( + 15% and + 30%) and decreased ( - 15% and - 3 0 % ) seepage. The downward seepage of a given fraction F is equal to or greater than WF. (C) The complementary fraction F ' for two different assumed levels of the critical downward seepage (30 and 50 mm. y r - ~).
P.E. V. van Walsum, J.H.J. Joosten /Agricultural Water Management 25 (1994) 45-55
53
conductivities of 0.71 and 0.12 mm- d - 1. This variation is the result of local differences in the type of peat and the degree of humification. For a critical downward seepage of 30 mm. yr 1, the calculated area with a less than critical seepage was about 38 ha (3% of the G r o o t e P e e l area); for a critical downward seepage of 50 m m . y r - 1 this area was about 88 ha (7% of the G r o o t e P e e l area). These values are similar to the actual area of rare groundwater-dependent vegetation types in the G r o o t e P e e l (Van Walsum, 1992). However, these areas only give an indication of the area that complies with a certain seepage criterion. Apart from a maximum level of downward seepage, the other hydrologic conditions for bog growth also have to be fulfilled (i.e. the shallow drainage base and the high local storage capacity of the substrate). Because these conditions will not be found everywhere, a certain reduction of the mentioned areas should be applied. This problem was circumvented by presenting all results as percentage changes of the area where the seepage criterion is fulfilled - the latter we called the "Potential Bog Area". The relative change appeared to be roughly the same for both 30 and 50 mm. yr 1 of downward seepage; thus the method does not require knowing the exact value of the critical downward seepage. In a number of scenarios (Van Walsum, 1990) the computed change of the "Potential Bog Area" has been plotted against the change of the downward seepage (of the G r o o t e P e e l as a whole). Fig. 3 shows a regression curve through the points. Less downward seepage, resulting in more stable water levels is also favourable for vegetation types that
APBA (%) 80,
75,
60,
45.
30 2
15'
O'
-15. I I I
-30 -25
I
I
I
I
I
I
I
-20
-15
-10
-5
0
5
10
AW (%)
Fig. 3. Approximaterelationship between the changeof the long-termaverage seepagefromthe Groote Peel as a whole (AW) and the "Potential Bog Area" (APBA).
54
P.E. V. van Walsum, J.H.J. Joosten /Agricultural Water Management 25 (1994) 45-55
are less demanding than living bog. Therefore, a reduction of downward seepage will lead to a general ecological upgrading of the area. For this reason the "Potential Bog A r e a " was used as an overall ecological standard for the G r o o t e Peel.
5. Concluding remarks A method has been presented for quantifying the local ecological impacts of regional hydrologic interventions on a degenerated bog. It was needed to bridge the gap between the spatial resolution with which the regional hydrologic model could be calibrated (subregions with an average area of 80 ha) and the resolution required for predicting the effect on the bog vegetation (a few hundred square metres, or even only a few square metres). In the future it will be possible to do regional hydrologic modelling with a very fine mesh, However, this is not only a matter of having enough computing power. More importantly, detailed hydrologic information about the situation in the field is necessary. Especially in landscapes with a large fine-scaled diversity, i.e. in bogs that have been subject to exploitation in the past, obtaining all these data in a non-destructive manner is an impossible task. For this reason, it will always be necessary to devise an approximate procedure for downscaling results of regional hydrologic modelling to local ecological impacts. Such a procedure will often only predict the (changing) area of valuable vegetation, without being able to pinpoint the exact spots where the changes will take place. Though the straightforward applicability to other types of hydrologic-ecological interactions may seem limited, the present case study does illustrate the kind of approach that could be followed. In the case of the G r o o t e P e e l this approach has played an important role in the decision-making process leading up to the designation of a 2000 m wide buffer zone (Vermeer and Joosten, 1992).
6. References DHV, 1989. Ecologische effecten van drainage op de Groote Peel. DHV Raadgevend Ingenieursbureau,Amersfoort (The Netherlands), 64 pp. (in Dutch) Joosten, J.H.J. and Bakker, T.W.M., 1987. De Groote Peel in verleden, heden en toekomst. Staatsbosbeheer, Utrecht (The Netherlands). Rapport 88-4. 291 pp. (in Dutch) Joosten, J.H.J. and Van Noorden, B.P.M., 1992. De Groote Peel: leren waarderen; Een oefening in het waarderen van natuurelemententen behoeve van het natuurbehoud. NatuurhistorischMaandblad,81 : 203-211. ( in Dutch) Landinrichtingsdienst, 1988. De waterhuishouding van de Groote Peel. Landinrichtingsdienst, Utrecht (The Netherlands), 24 pp. (in Dutch) Poelman, A., 1987. Geohydrologische modelstudie van de Groote Peel en omgeving. Staatsbosbeheer, Utrecht (The Netherlands). Rapport 88-3, 60 pp. ( in Dutch) Poelman, A. and Joosten, J.H.J., 1992. On the identification of hydrological interaction zones for bog reserves. In: O.M. Bragg, P.D. Hulme, H.A.P. Ingram and R.A. Robertson (Editors), Peatland Ecosystems and Man: An hnpact Assessment. Dept. of Biological Sciences University of Dundee, Dundee, pp. 141-148. Projectgroep de Groote Peel, 1990. Technische maatregelen ter verbetering van de waterhuishoudingin de Groote Peel en hun effecten. Ministerie van Landbouw, Natuurbeheer en Visserij, Den Haag (The Netherlands), 63 pp. (in Dutch)
P.E. V. van Walsum, J.H.Z Joosten / Agricultural Water Management 25 (1994) 45-55
55
Querner, E.P., 1988. Description of a regional groundwater flow model SIMGRO and some applications. Agric. Water Manage., 14: 209-218. Van Walsum, P.E.V., 1990. Waterbeheer rondom de Groote Peel; verkenning en evaluatie van scenario's. DLO Staring Centrum, Wageningen (The Netherlands). Rapport 106, 106 pp. (in Dutch) Van Walsum, P.E.V., 1992. Water management in the Groote Peel bog reserve and surrounding agricultural area; simulation and optimization. DLO Winand Staring Centre, Wageningen (The Netherlands). Report 49, 124 pp. Vermeer, J.G. and Joosten, J.H.J., 1992. Conservation and management of bog and fen reserves in the Netherlands. In: J.T.A. Verhoeven (Editor), Fens and Bogs in The Netherlands: Vegetation, History, Nutrient Dynamics and Conservation. Geobotany, 18. Kluwer Academic Publishers, Dordrecht, pp. 433-478.