The average annual cost corresponds to $568,371.3 and this value was used as the basis of the costs to be recovered through the collection. These costs were unknown and this study has provided a first estimate of these values. Few resources were directed to the stormwater systems until 2014. By contrast, there was an increase in 2015 and 2016. Despite this, there was a greater exchange variation in the currency conversion of this period. A consideration of the full financial and economic costs of stormwater is more comprehensive, but data are not available in this case. Besides that, annual budgets should include adequate staffing and financial resources to conduct the maintenance activities needed to assure that stormwater systems operate properly. However, since stormwater is not regulated and there are no specific resources, there is also no specific staffing. ### Benchmarking affordability Given the concern about users' ability to pay, an analysis was carried out in order to quantify affordability at full cost recovery rates. Income-based benchmarks are more popular for such quantification (Mack & Wrase 2017). In Brazil, there is no national representative database on stormwater prices. This makes level assessments of affordability challenging. For this reason, the price of other sanitation services was used as a basis. The US Environmental Protection Agency (EPA) proposes that households spend no more than 4.5% of their median household income on water and wastewater services. This value is similar to that income benchmark recommended (5%) by World Bank (Mack & Wrase 2017). Expenses higher than this amount are considered unaffordable. Brazilian households from the southern region (study area) spend 0.9% of their monthly income on these services, the same value as the Brazilian average (IBGE 2010). In this study, affordability was assessed similarly to the EPA's affordability benchmarks, adapting the values to the Brazilian reality. The question of whether such Brazilian value can be considered desirable or undesirable is beyond the scope of this article. The same importance for stormwater bills was adopted in this study. Thus, bills that constitute more than 0.9% of the median household income are considered unaffordable. For this analysis, the fee, which had previously been calculated per parcel, was grouped by census block, generating an average fee for each sector. A census block is a territorial unit established for cadastral control, formed by a continuous urban or rural area, which contains a certain quantity of households. This is the smallest unit of analysis that can be discretized through official census data in Brazil. The most recent data available refer to the year 2010, the last national census year (IBGE 2011). The costs of stormwater services refer to the period 2010–2017, so the income data were adjusted to 2017 according to the increase of the minimum wage in this period (83.73%). Impervious land occupies approximately 10.60% of the study area; this can be considered of low-intensity development (Figure 2). The impermeable areas in the parcels, input parameters for the fee application, represent 7.5% (2.03 km2). It was possible to identify 4,951 lots (72%) with some type of impervious areas, while 1,913 lots were vacant (28%). So, there is no calculation basis for collection in empty lots. The SERU estimate also considers this kind of parcel in the calculation (total of urban lots in the city in Equation (4)), since they can be quickly occupied given the current dynamics related to the growth of cities. However, the lots effectively occupied are charged for the operation and maintenance of stormwater services (Equations (5) and (6)). Figure 2 Location and land-use map of urban area of Santo Amaro da Imperatriz, Brazil. Figure 2 Location and land-use map of urban area of Santo Amaro da Imperatriz, Brazil. Close modal Fee collection in vacant lots is common in the international scenario because the ‘non-polluting’ users also benefit from the stormwater services. Despite this, the Simplified ERU methodology disregards this collection because it is difficult to convince these users to contribute to environmental improvement when its degradation is not directly imputed to them. Otherwise, there are examples of the taxation of vacant lots in order to encourage their occupation. Vacant lots in urban areas encumber the State, which is forced to spend resources on new urbanization to accommodate city growth, while vacant lots fuel real estate speculation. Consideration or not of the taxation of vacant lots must be a decision made by each municipality in agreement with its urban policies. It was possible to identify 76 industrial buildings distributed in 52 lots, totalling 0.13 km2 of impervious area. The SERU (Equation (4)) was estimated at 294.32 m2, considering industrial users, or 277.57 m2 when analysing only the residential areas. This is a small difference (16.75 m2) and it confirms that simplification of the charge classes for residential lots can be carried out in small municipalities with low-intensity development. The SERU value (294.32 m2) is very close to the average impervious area from the United States, which is 269.42 m2 (Campbell et al. 2016), although the stormwater services provided are very different. The annual fee is$82.80 by SERU (Figure 3), which is equivalent to $6.90 monthly. Thus, each resident must pay$0.28 for each m2 of impervious area. This value is within the range of international fees ($0.02–$1.51/m2) shown by Tasca et al. (2017). However, different types of cities with different mixes of land uses, topography and population densities will have vastly different outcomes. Research in the Greater Melbourne region in Australia, for example, showed an average value of $0.86/m2 and$583/property, constituting the impervious area tariffs (Coombes 2018b). This author utilized a combination of local and regional costs of SWM that includes infrastructure, amenity, waterway health, and recreation actions, that is, this fee has been set to account for the revenue required to pay all costs of SWM throughout the region. On the other hand, SERU has been set to account for the revenue required for pay operations and maintenance costs of stormwater and its total revenue was $568,371.30. Most lots (67%) must pay up to 1 SERU, while 23% must pay between 1 and 2 SERUs. Figure 3 Annual fee ranges by parcel. Figure 3 Annual fee ranges by parcel. Close modal The distribution of impermeable areas by parcel versus annual fees can be analysed as in Figure 4. Due to the SERU design, there is a direct relationship between these variables: as one variable increases, the other also increases. In these instances of prediction, higher impermeable area values led to higher annual stormwater fees. Figure 4 Impermeable areas by parcel x annual fees, with emphasis on more and less impervious lots. Figure 4 Impermeable areas by parcel x annual fees, with emphasis on more and less impervious lots. Close modal There are a few lots with high payments. The highest annual fee ($4,437.08) refers to a lot with 53.58 SERUs, a Municipal Exhibition Park. It was considered that public areas should also be charged according to their impervious areas, but it is not the purpose of this study to discuss who should pay this fee. The lowest charge refers to a small residential lot (0.20 SERU), which is also charged because there is no minimum reference unit. Any kind of decision about who must pay can be easily incorporated into the calculation methodology once the database is already prepared. In this sense, the methodology is adaptive to the planning process.

Santo Amaro da Imperatriz has 22 census blocks in the urban area, totalling 4,671 households and 14,970 residents. The median monthly income of the urban area is $1,434.19 and the standard deviation is$420.36. The variation is 960.99 (Sector 18) to 2,423.43 (Sector 2) as shown in Figure 5. The annual fees shown in Figure 3 were transformed into monthly fees and were grouped by census block, generating a monthly average fee for each sector. This database was crossed with the median monthly incomes (Figure 5(a)), which makes it possible to quantify the affordability at full cost recovery rates (Figure 5(b)). Just two sectors (21 and 22) will face affordability challenges to payment. This means that 323 households will have to allocate monies from other expenses to pay for stormwater services. Although this may not be a problem for higher-income households, this is an issue for low-income households and those in poverty who barely make enough money to pay for basic living expenses. In order for these households to spend no more than 0.9% of their monthly income, the monthly fee would have to be reduced by 22% and 36% in sectors 21 and 22, respectively.

Figure 5

(a) Average monthly income by census block in the urban area. (b) Impact distribution by census block.

Figure 5

(a) Average monthly income by census block in the urban area. (b) Impact distribution by census block.

Close modal

### Critical analysis of the SERU methodology – limitations

The Simplified ERU method uses the concept of the total impervious area (TIA), which is widely used as an indicator of environmental quality (Arnold & Gibbons 1996). However, this concept has some limitations. TIA represents the fraction covered by construction, that is, non-infiltrating surfaces (concrete, asphalt, and buildings). Hydrologically, this definition is incomplete for two reasons (Booth & Jackson 1997):

• 1.

TIA ignores pervious surfaces that are sufficiently compacted or otherwise so low in permeability that the rate of runoff from them is similar to or indistinguishable from pavement.

• 2.

TIA includes some paved surfaces that may contribute nothing to the storm-runoff response, such as gazebos, tents, wooden structures suspended from the ground. Indeed, the effective impervious areas (EIA) – impervious surfaces with a direct hydraulic connection to the stream networks via stormwater infrastructure – are really contributing to runoff and their use may result in overestimation of runoff volumes and stormwater fees.

EIA is an integrative measure characterizing urbanization, too, but it is not easily quantifiable because it requires information on the connectivity of impervious surfaces (Bell et al. 2016). So, it is complicated to identify TIA and EIA and estimate their contribution to the runoff, although there are some studies designed for this. These approaches were not addressed in the SERU method. The collection under the TIA concept does not encourage the adoption of stormwater control measures (SCMs) since it charges the same for all lots. Selection of the appropriate SCM is dependent on local environmental conditions, and economic incentives and fee discounts should therefore be analysed for each region. In addition, SCMs implementation is related to SWM regulation. SERU considers locations without regulation. However, these incentives can be applied at any time and each property owner can choose to pay the stormwater fee or to reduce the volumes of stormwater runoff discharging from their parcel.

Besides that, many urban applications, including impervious surface and building infrastructure mapping, require a high to moderate temporal resolution of from 1 to 5 years. These types of applications also utilize high spatial resolution, often at or below 1 × 1 m, in order to capture the detail and complexity of the urban landscape (Jensen & Cowen 1999). Many municipalities, especially the smaller ones, may not have data with this level of detail. Lastly, the SERU method does not consider the municipal roads runoff management because this service may be embedded into other rates (generally as part of road budgets), as it happens in Brazil. Besides this, the individualization of all beneficiaries (residents and non-residents) is complex and other options, such as a fixed fee, should be evaluated.

This study presented a stormwater fee model developed for small municipalities to finance the operations and maintenance costs of stormwater services. The objective was to generate funding to solve the current stormwater problems mainly where services are not well delineated, and to be self-supporting. In this way, the main method (Equivalent Residential Unit – ERU) from the United States was simplified and based on the amount of impervious area of all parcels located within the urban area. The Simplified ERU has a single billing class, simplifying the administration of local planners and it is especially important for cities that do not have a specific sector for SWM.

The Simplified ERU was simulated in Santo Amaro da Imperatriz, a small city in the southern region of Brazil, where there is no attention to drainage systems because adequate resources and financing are not available. Just two sectors faced compounding economic factors that have an impact on their ability to pay for water services. This method was shown to be a feasible and rapid method for the design of service fee structures. Thus, given its simplicity, it could be reapplied in small municipalities that do not have a complex SWM. Besides being simple and relatively easy to apply, it requires few resources for its development. It can be easily adapted according to the peculiarities of each city. Changes in the type of cost considered, billing classes, fee discounts, for example, can be applied at any time. Stormwater fees are necessary to maintain the public stormwater system, and represent an equitable way for the community to share the cost of a public service in order to reduce the impacts of urbanization. A broad scientific discussion can contribute to the widespread dissemination of the importance of a stormwater fee system and its benefits.

The authors gratefully acknowledge the financial support given by CAPES (Coordination and Improvement of Higher Level or Education Personnel), and CNPQ (National Council of Scientific and Technological Development). We thank the anonymous reviewers for their constructive comments that contributed to improve the manuscript.

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