Dimensioning of Buffer Zones and Buffer Strips

The dimensioning of riparian buffer zones and riparian buffer strips is based on three functions of riparian buffer ecosystems: filtering polluted overland flow from

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Distance from the forest edge (m)

Distance from the forest edge (m)

Figure 12 The filtering effect of riparian gray alder (Alnus incana) forests through the application of fertilizers from airplanes. In this case, carbamide (urea; NH4CONH2) was used to fertilize the winter barley fields. Wind direction in the series: 1 and 2 - perpendicular to the forest, 3 - perpendicular from the forest, 4 - parallel to the forest edge, with amount of carbamide washed out from the canopies with rain: A - first, * - second, • - third, and © - fourth series of experiments. Adapted from Mander U (1995) Riparian buffer zones and buffer strips on stream banks: Dimensioning and efficiency assessment from catchments in Estonia. In: Eiseltova M and Biggs J (eds.) IWRB Publication No. 37: Restoration of Stream Ecosystems, pp. 45-64. Slimbridge, UK: International Waterfowl and Wetlands Research Bureau (IWRB).

intensively managed adjacent agricultural fields avoiding intensive growth of aquatic macrophytes via shading by canopies, and filtering polluted air from local sources of pollution (i.e., agrochemical treatment of fields using air planes or helicopters).

Although the buffering efficiency of vegetated riparian buffers has been reported by many authors for several ecosystems, there are a few examples in the literature of the dimensioning of buffer zones and strips. The first experiences have been obtained from the USA, where methods for the determination of the parameters of buffer zones adjacent to feedlots and manure land treatment sites have been developed. Phillips presented calculations to evaluate the buffering effectiveness of riparian forests along a coastal plain river. However, calculation methods usable for landscape planning and stream restoration pur poses are not well developed. It has been found that the purification of the overland flow within the buffer strips has a nonlinear character: much more organic matter, nitrogen, and phosphorus was removed in the upper part of the strips than in the downslope part (Figures 7 and 8).

The calculations presented are based on hydrological models which appeal to the capacity of buffer strips to infiltrate overland flow, and are supplemented with parameters significant for the absorption and cation exchange capacity of soils. Basically, this method is com parable to the widely used universal soil loss equation (USLE). However, the length slope factor in this equation, which is a purely empirical relationship, does not account for changes in either surface flow or erosion processes. Therefore, a specific slope length factor has been included in our equation. This refers to overland flow concentration in lower parts of the relief, for example, in gullies.

The width of riparian buffer strips depends on the soil and relief conditions of the adjacent landscape, and nor mally lies between 5 and 50 m. This can be determined on the basis of maps of reclaimed areas at a scale of 1:2000 with detailed topographic and soil data, using the following formula applied in the planning of buffer zones in Estonia

[2l where P is the optimal width of forest/bush buffer strip (m); q is the mean intensity of overland flow during the thawing period (mm d 1 for Estonia q = 8.4); f is the specific slope length (m); i is the mean slope in the catchment (i = tan a); m is the roughness coefficient of the surface in the catch ment (mean value for ploughed fields: 1.0, for intensively managed grasslands: 1.1, for natural meadows: 1.2); K is the water infiltration within the buffer strip during the spring (mmmin 1 mean value over different soil types normally varies between 0.1 and 1.0); n is the soil absorption capacity; and the constant 0.000 69 is the time variation coefficient (from days to minutes).

In the case of a heterogeneous topographic situation (e.g., moraine hilly landscapes), the specific slope length f is defined as f = FIl

[Bl where F is the elementary catchment area of a gully (m2) and l is the width of a gully immediately on the bank of a stream or on the lakeshore (m). This approach underlines buffer strips' necessity on gullies. For homogeneous slopes, the value of fwill be calculated as the distance (m) from the watershed border to the stream bank (lakeshore).

The soil absorption capacity n is calculated as ln Isx ln Isc

[4l where ln Isx is the specific area of the investigated soil type (m2 g J) and ln Iscoarse sand is the specific area of the coarse sand (m g ).

The mean values of the integrated soil parameter k (k = Ki x n) for the main soil types are given as: coarse sand: 1.00, fine sand: 0.80, loamy sand: 0.61, sandy loam: 0.53, sandy clay loam and loam: 0.43, clay loam and sandy clay: 0.33, clay: 0.21.

Using the formulas presented above, a nomograph for the estimation of the recommended width of riparian buffer strips has been compiled (Figure 13).

Riparian forest buffer strips can also be dimensioned on the basis of the shading effect. To generalize the factors influencing the dynamics of shading ratio (Sn; Figure 11) at the water surface in watercourses, the following equa tion based on the contributions of overstory vegetation shading is presented:

where D is the closeness of the canopy (%), W is the width of the watercourse (m), T is the height of the vegetation (m), Z is the angle between vertical plane and the line oriented to the Sun (in degrees;

Figure 13 The nomograph for the determination of the width of the recommended buffer strip. Adapted from Mander U, Kuusemets V, Lohmus K, and Mauring T (1997) Efficiency and dimensioning of riparian buffer zones in agricultural catchments. Ecological Engineering 8(4): 299-324.

Figure 14 Scheme and nomograph to estimate optimal parameters of streamside vegetation to stream surface shade. Adapted from Mander U (1995) Riparian buffer zones and buffer strips on stream banks: Dimensioning and efficiency assessment from catchments in Estonia. In: Eiseltova M and Biggs J (eds.) IWRB Publication No. 37: Restoration of Stream Ecosystems, pp. 45-64. Slimbridge, UK: International Waterfowl and Wetlands Research Bureau (IWRB).

Figure 14 Scheme and nomograph to estimate optimal parameters of streamside vegetation to stream surface shade. Adapted from Mander U (1995) Riparian buffer zones and buffer strips on stream banks: Dimensioning and efficiency assessment from catchments in Estonia. In: Eiseltova M and Biggs J (eds.) IWRB Publication No. 37: Restoration of Stream Ecosystems, pp. 45-64. Slimbridge, UK: International Waterfowl and Wetlands Research Bureau (IWRB).

see Figure 14), A — R is the orientation of the stream stretch (in degrees), Y is the distance from the water table to the forest/bush (m), and C is the coefficient, depending on the form of tree crowns and their close ness; C = 0 in the present study, which deals mostly with dense stands of gray alders and willows.

In eqn [5], the shading ratio Sn shows the dynamics of shaded area on the water surface (%) over a certain period. The most dynamic parameter Z can be deter mined on the basis of the local values of the latitude and the dynamics of the declination and height of the Sun. As an example, a nomograph has been compiled for Tartu, Estonia (58°23' N; 23°44' E) at noon on the summer solstice (21 June) (Figure 14). It shows, for instance, that the 50% shading value of a 1 m wide west-east oriented stream will be guaranteed by a 5 m high alder forest buffer strip (canopy closeness D = 100%, C = 0) standing at a distance of 2.5 m from the stream.

This method is useful in landscape planning in the case of larger streams, in order to determine the optimal para meters for designed streamside vegetation. It is common practice in land reclamation for the planning of forest buffer strips on stream banks to be determined on the basis of their buffering capacity.

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