Flood regulating ecosystem services—Mapping supply and demand, in the Etropole municipality, Bulgaria

Abstract

Floods exert significant pressure on human societies. Assessments of an ecosystem's capacity to regulate and to prevent floods relative to human demands for flood regulating ecosystem services can provide important information for environmental management. In this study, the capacities of different ecosystems to regulate floods were assessed through investigations of water retention functions of the vegetation and soil cover. The use of the catchment based hydrologic model KINEROS and the GIS AGWA tool provided data about peak rivers’ flows and the capability of different land cover types to “capture” and regulate some parts of the water. Based on spatial land cover units originating from CORINE and further data sets, these regulating ecosystem services were quantified and mapped. Resulting maps show the ecosystems’ flood regulating service capacities in the case study area of the Malki Iskar river basin above the town of Etropole in the northern part of Bulgaria. There, the number of severe flood events causing significant damages in the settlements and infrastructure has been increasing during the last few years. Maps of demands for flood regulating ecosystem services in the study region were compiled based on a digital elevation model, land use information and accessibility data. Finally, the flood regulating ecosystem service supply and demand data were merged in order to produce a map showing regional supply-demand balances. The resulting map of flood regulation supply capacities shows that the Etropole municipality's area has relatively high capacities for flood regulation. Areas of high and very high relevant capacities cover about 34% of the study area. The flood regulation ecosystem service demand map shows that areas of low or no relevant demands far exceed the areas of high and very high demands, which comprise only 0.6% of the municipality's area. According to the flood regulation supply-demand balance map, areas of high relevant demands are located in places of low relevant supply capacities. The results show that the combination of data from different sources with hydrological modeling provides a suitable data base for the assessment of complex function–service–benefit relations.

Introduction

Ecosystems regulate essential ecological processes and life-supporting systems through bio-geochemical cycles and other natural processes (Daily, 1997, de Groot et al., 2010). The concept of ecosystem goods and services proposes an appropriate methodological framework to analyze these relations. Therefore, ecosystem services have become a very popular scientific topic, especially during the last two decades (Burkhard et al., 2010). The Millennium Ecosystem Assessment defined ecosystem services as “the benefits that people derive from ecosystems” (MA, 2005). de Groot et al. (2010) grouped them into the four broad categories: provisioning services, regulating services, habitat or supporting services and cultural & amenity services.

Besides a broad range of scientific publications about the further conceptualization of ecosystem services (Fisher and Turner, 2008, Boyd and Banzhaf, 2007, Chee, 2004), there is a clear lack of spatially explicit service assessments at regional, national and continental scales (Daily and Matson, 2008). Only few studies analyze the capacities of landscapes to supply services. But, resource managers need easily understandable, spatially explicit tools for management and trade-off decisions at landscape scales (Kienast et al., 2009). Therefore, we apply a spatially explicit concept that links biophysical conditions to ecological functions, ecosystem services and human benefits, using the example of flood regulation in a Bulgarian case study region.

Floods are among the most dangerous natural disasters that threaten large territories in Bulgaria (Nojarov, 2006). Mountain areas are even more vulnerable to that threat, because, they are formed in relatively small watersheds, the rivers’ peak flows move very fast along the river bed. There are different kinds of floods and those which are typical for Bulgaria can be grouped in two main categories according to their formation:

• rainy-fluvial floods are formed by torrential rains which cause rivers to rise quickly creating a significant overflow beyond the river banks and

• snow melting-fluvial floods producing the same effect but formed by both rains and melting snow.

The local authorities and civil defense organizations have limited options for reaction when such hazardous events occur. Therefore, flood prevention and mitigation measures are very important. The conventional practice is to build river protection dikes and other hydro-technical equipment in the vulnerable areas. But, the potential of natural landscapes to mitigate the negative effects of this extreme phenomenon is usually neglected (Jonkman et al., 2004).

If ecosystem services are the goods that people derive from nature, hazardous flood events are often considered as being part of the “bads”, the ecosystem disservices (Lyytimäki et al., 2008). This is especially true in areas where people settled too close to water bodies or constructed their properties in previous flood plains. Additional problems arise when people actively modify water bodies, watersheds, or flood plains. Hence, most of the flood-related disservices are ecosystem functions created by human activities. On the other hand, flood protection is one of the most important regulating ecosystem services that may increase or reduce the negative effects of water-related disasters. In the ecosystem service classification scheme suggested by de Groot et al. (2002), flood-related ecosystem services are “flood prevention” and “drainage and natural irrigation”. As related ecosystem processes and elements, “ecosystem structure's influence on dampening environmental disturbances” and “the role of land cover in regulating runoff and river discharge” are mentioned (de Groot et al., 2002). Forests especially provide natural hazard mitigation and water regulation services by reducing flood-danger, preventing damage to infrastructure and influencing water retention capacities (de Groot et al., 2010).

The human benefit of flood regulating ecosystem service provision is flood-damage mitigation and finally, the protection of human properties (Fisher et al., 2009, Boyd and Banzhaf, 2007). The latter being part of the demand side of ecosystem services. Fisher and Turner (2008) state that ecosystem services just contribute to other flood mitigation measures based on e.g. capital or dykes. In Chee (2004), flood regulation is based on “moderation of weather events, regulation of the hydrological cycle and maintenance of coastal and river channel stability” which all are part of the “stabilizing ecosystem services”.

The regulating role of wetlands, floodplains and coastal ecosystems is usually emphasized (Ming et al., 2007, Posthumus et al., 2010) but it is also important to pay attention to the functions of other ecosystems throughout river basins which control the processes of water balance (Pert et al., 2010). For that reason, we have to separate service production (supply) areas from service benefit (demand) areas (Fisher et al., 2009). The ecosystems affect the water balance mainly through two processes: interception and infiltration. Interception depends on the structure of the ecosystem above ground (land cover) while the infiltration is strongly determined by the soil properties. The surface runoff, which is the main factor for flood formation, also depends on abiotic factors like rocks and topography.

Regulating ecosystem services can have preventive or mitigating functions. In the first case, the ecosystems (i.e. forests) redirect or absorb parts of the incoming water (from rainfall), reducing the surface runoff and consequently the amount of river discharge. This ecosystem service plays its role before flood occurrence and in some cases it can even prevent it. This is valid especially for the rainy-fluvial flood type, while for snow melting-fluvial floods the ecosystems’ prevention capacity is far lower. One role of forests in mitigating flooding in the case of melting snow is the reduction of wind velocity and delay of snow melt caused by warm winds (e.g. foehn). However, the more important flood mitigation function comes into effect when the flood is already formed. The ecosystems (i.e. flood plains and wetlands) provide retention space for the water surplus to spill, thus reducing the flood's destructive power.

Hence, flood regulating ecosystem service assessments should conform to the biophysical characteristics and the likelihood of a flood in the particular area. There has been a significant increase in the number of extreme rain events (Velev, 2005, Bocheva et al., 2009) and thus, disastrous floods in Bulgaria (Nikolova, 2001, Nikolova et al., 2008) and other European countries (Kundzewicz et al., 2005, Kundzewicz et al., 2010, Barredo, 2007, Lugeri et al., 2010) caused by torrential rains during the last decades as well as the damages and casualties caused by them (Loster, 1999, Jonkman, 2005). The area of the Etropole municipality is among the most seriously affected in Bulgaria by these hazardous events. Creating a high demand for flood regulating ecosystem services, making it an appropriate case study example.

Different ecosystems have different functions and therefore also different capacities to provide ecosystem services. Following the concept for land-cover based ecosystem service assessments suggested by Burkhard et al., 2009, Burkhard et al., 2012, ecosystem service supply capacities were assessed for 12 different land cover types occurring in the study region. These capacities represent the current state of flood regulation performance as indicated by the model simulations and data analyses (capacities are not the same as potentials or option values in this case). Using the models described above we provide an assessment matrix which links the different land cover types in the study area and their biophysical attributes to their capacities to provide flood regulating ecosystem services.

Mapping of ecosystem services has been mentioned as one of the main challenges for the ecosystem service concept's implementation into decision making (Daily and Matson, 2008). Several promising approaches to spatially analyze landscape functions and ecosystem services have been published recently (Blaschke, 2006, Burkhard et al., 2009, Burkhard et al., 2012, Egoh et al., 2008, Kienast et al., 2009, Naidoo et al., 2008, Tallis and Polasky, 2009, Troy and Wilson, 2006, Willemen et al., 2008, Haines-Young et al., 2012). Due to the immense complexity of ecosystems and their service provision, all mapping methods at hand (including the one presented here) are still in the development and testing phase. Moreover, all studies mentioned above differ with regard to their methods of ecosystem service evaluation, the selection of ecosystem services to be assessed, and the spatial scale to which they refer (see Burkhard et al., 2009 for a short review).

Mapping with focus on flood regulating ecosystem services has been rather rare up to now. In many cases flood regulation has been assessed together with other ecosystem services (e.g. Egoh et al., 2008, Posthumus et al., 2010) or in connection with the derivation of risk maps of flood and earthquake hazards over Europe (Schmidt-Thomé et al., 2006). Ming et al. (2007) created maps of water balance-related ecosystem functions and flood mitigation ecosystem services for wetland soils in a case study in China. Syrbe and Walz (2012) presented a map of Service providing areas (SPA), service benefiting areas (SBA) and service connecting areas (SCA) for the flood regulating service in Saxony (Germany). The study presented here is one of the first ecosystem service assessments exclusively focusing on flood regulating ecosystem services on a landscape scale.

In addition to the flood regulating ecosystem services supply capacities’ assessment, we will integrate the demand for flood regulation in the study region as well. The demands for flood regulation are linked to the benefits that people obtain by this service. In our case, benefits are the protection of property such as houses infrastructure, farmlands and of course, human life. If efficient natural flood protection and mitigation are to be achieved in this region, the supply of flood regulating ecosystem services by nature on the one hand should spatially match the demands of society on the other hand. This is especially interesting in the case of flood regulation as related services have to be provided in regions which are directly linked to the area where the demand is located, for example along the same watercourse or within the same watershed. In contrast to many other ecosystem services, flood regulating services cannot be imported from other regions. In the case of flood regulation, the service production areas (after Fisher et al., 2009) have to be physically linked to the service benefit area. Thus, there must be a close connection between the area of service supply and service demand. Water retention in regulating ecosystem service supply areas prevents excessive water flows during flood events, providing direct benefits to people living in affected regions. Forests for example, which in our case study are located in mountainous areas within the same watershed, contribute greatly to flood regulation which protects settlements further downhill. By merging the maps of flood regulating ecosystem services supply and demand, regional patterns and balances between supply and demand can be visualized (Burkhard and Kroll, 2010, Burkhard et al., 2012).

Appropriate indicators that represent quantitatively the processes by which ecosystems regulate water balance are needed in order to assess the capacity of ecosystems to prevent and mitigate floods. They can help to determine quantitative relationships between the various steps of service provisioning and how to measure the benefits derived from ecosystem services (van Oudenhoven et al., 2012). de Groot et al. (2010) propose the use of state indicators for natural hazard mitigation services such as “water storage capacity (buffer) in m³” and performance indicators like “reduction of flood danger and prevented damage to infrastructure”. Water storage capacity is a good indicator for the damage mitigation function of floodplains and wetlands and their spatial dimensions can be measured. However, it would be more difficult and uncertain to derive such an indicator for the prevention function of ecosystems like forests or grasslands. This is so due to the fact that their regulation function depends not only on their storage capacity but also on a number of other factors and functional processes such as interception and infiltration, surface parameters like roughness and slope as well as external factors like rainfall quantity and intensity, seasonal state of the vegetation and initial soil saturation. The use of catchment based hydrologic models provides the basis to reveal the varying importance of factors and processes responsible for the formation of river swellings as well as the capability of different land cover types to “capture” part of the incoming water, which reveals their regulation capacity. The GIS based AGWA (Automated Geospatial Watershed Assessment) tool and its constituent models KINEROS (KINematic Runoff and EROSion model) and SWAT (Soil and Water Assessment Tool) have been used for flood hazard assessment in two case study areas in the Stara Planina Mountains (Nedkov and Nikolova, 2006, Nikolova et al., 2007, Nikolova et al., 2009, Nedkov, 2010) as well as to determine the influence of land cover changes on flood formation processes (Vatseva et al., 2008). Here we use these models to further develop the indicators for the capacity of flood regulation ecosystem services.

The main objectives of this article are:

• to utilize hydrologic modelling to identify and assess flood regulating ecosystem services in the case study area,

• to define the capacity of different land cover types in the study area to contribute to flood regulation,

• to define areas of flood regulation ecosystem services’ demands, and

• to further develop the concept of mapping of ecosystem services.