Modelling Stormwater Runoff Changes Induced by Ground-Mounted Photovoltaic Solar Parks: A Conceptualization in EPA-SWMM

The continuous growth of global population causes increasing concerns on food, water, and energy sectors (Sarkodie and Owusu 2020; Makaronidou 2020). The energy generation processes are facing major challenges such as sustainability, cost, security, and market price fluctuations (Almomani 2020; Ebhota and Jen 2020). In addition, the increase in environmental awareness and the application of more stringent discharge regulations has directed the scientific community to work on developing alternative, sustainable, and renewable energy sources (Ahmad et al. 2020; Tawalbeh et al. 2021; Yavari et al. 2022; Bertsiou and Baltas 2022; Loucks 2023).

Among all the renewable energy sources, solar photovoltaic (PV) is one of the most widespread in the word (Ravi et al. 2014; Armstrong et al. 2016; Barron-Gafford et al. 2016; Hassanpour Adeh et al. 2018). Although solar energy is universally recognised as environmentally friendly energy source, impacts on surface hydrology of large parks have not been comprehensively addressed in literature (Turney and Fthenakis 2011; Pisinaras et al. 2014; Yavari et al. 2022). With growing concern over the impact of land use changes on stormwater runoff, the construction of large-scale solar power plants may face obstacles in the future unless appropriate quantification of this impact is addressed and proper measures are taken to mitigate potential increment of flow peak and volume discharge (Turney and Fthenakis 2011).

Assessment of runoff generation in PV solar parks can be carried out by modelling-based approaches, that have the advantage, with respect to purely experimental studies, to allow the investigation of the influence of different hydrological conditions (Yavari et al. 2022). For instance, among the studies based on such approach, Barnard et al. (2017) set up a 1D/2D model by coupling Flo-2D and HEC-HMS to simulate stormwater runoff at three selected solar PV installations in west Texas. However, no comparison with the pre-installation scenario has been carried out thus preventing the possibility to evaluate the impacts on stormwater runoff induced by the presence of solar panels. HEC-HMS was also used to study hydrologic dynamics in a Nevada solar farm (Edalat 2017). The simulations showed that runoff volume always increases after solar panels installation. However, one major limitation of this study was that solar panels were represented as an impervious surface on the ground, and simulation of the infiltration process could not be permitted under the panels, as it would indeed occur at an actual site. Thus, this approach likely overestimates runoff volume. The Soil & Water Assessment Tool (SWAT) was used for assessing the impact of PV solar parks on watershed hydrology by Pisinaras et al. (2014). Solar parks installation was represented by implementing in the model soil physical properties/ground cover changes, curve number increases associated with imperviousness, and reduced solar radiation. In this case, model limitations consist in the fact that the dynamics of the runoff formation in the solar park are not explicitly taken into account. Other researchers developed a custom-built model for representing runoff in solar parks (Cook and McCuen 2013). The results indicated that the addition of solar panels over a grassy field does not change the volume of runoff, the peak discharge, nor time to peak. More recently, Wang and Gao (2023) conducted experiments at the plot-scale to investigate impacts of PV panels on rainfall-runoff and soil erosion processes. Results showed that runoff volume, peak flow discharge rate and overland flow velocity are not remarkably impacted by the presence of PV panels. However, further investigations are needed to transfer the obtained results at the plot scale to a real scale solar park.

Analysis of the literature on the topic highlights a research gap consisting in the lack of a comprehensive tool for the assessment of the impacts of real-scale solar parks on stormwater runoff, by taking into account the hydrological processes occurring within the park and all the variables affecting the park response to precipitation events. In this paper, with the aim of overcoming many of the mentioned limitations within previous studies, we propose a novel conceptualization of PV power parks response to precipitation events capitalizing on the use of the free and open-source Storm Water Management Model (SWMM) (Rossman 2015). The conceptualization allows to take into account the complex hydrological process occurring in the solar parks during precipitation events and to assess how the process of runoff in the park is affected by the extension of the PV installation, soil properties and the characteristics of the rainfall events. Moreover, effects of long term changes in roughness surface induced by the presence of the panels can be taken into account in the analysis. We demonstrate the potentialities of the proposed approach considering a layout of the PV installation (panels size and inclination) as well as characteristics of the precipitation events that are encountered in Sicily (south Italy).

A total of 135 simulations have been run for the short-term condition by combining all the model parameters and precipitation inputs described in the methodological section.

Figures from 3 to 5 compares the outflows from the PV solar park and from the reference catchment as resulting from 9 simulations.

In order to assess the impact on runoff of the park extension, Figs. 3a, 3b and 3c show outflows from parks of extension 1, 2 and 3, respectively, and soil type C. In the 3 simulations, the precipitation input is a Chicago hyetograph of 1 hour duration and 20-years RP.

For fixed soil type and precipitation input, increments of the park extension result in increments of the peak flow and of the total runoff volume (Fig. 3a, b and c). In particular, peak flow per unit of width increases by 33% each time the area of the solar park is doubled (from extension 1 to 2 and from extension 2 to 3), due to the non-linearity of the processes involved in the runoff formation. The outflow curve from the solar park follows the corresponding curve from the reference catchment both in the rising and in the recession limb of the hydrograph, regardless of the park extension. Minimal differences between the two outflow curves can be observed at the beginning of the precipitation event (first 20 min), with values of the outflow from the solar park slightly higher than those from the reference catchment. Analysis of the simulations output allows to ascribe this behaviour to the most downstream part of the solar park. Indeed, the generic corridor in the solar park receives in input the direct rainfall and the runoff from the panel area. The two contributions are practically simultaneous as the time of concentration of the panel area is in the order of a few seconds. Therefore, for a given soil infiltration capacity, the corridor generates excess of runoff with the respect to the portion of the reference catchment placed at an equal distance from the outlet. This excess of runoff can be later infiltrated in downstream under-panel areas and corridors. However, the most downstream corridor delivers the runoff directly to the outlet thus anticipating the time for the beginning of the runoff as observed in Fig. 3a, b and c.

Influence of the soil type on the runoff from the solar park is shown in Figs. 4a, 4b and 4c, referring to simulations run for solar park of same extension (3) and precipitation input (Chicago hyetograph 3-hours duration, 50-years RP) but different soil type (A, B and C, respectively).

For fixed solar park extension and precipitation input, changing from sandy to clay soils results in increased peaks flow and total runoff volumes, due to the reduction in infiltration capacity (Fig. 4a, b and c). In particular, peaks flow from solar park results equal to 0.2, 2.5 and 2.8 L/s per unit of width for soil type A, B and C, respectively. Peak flow and total runoff volume from the solar park are greater than the corresponding values from the reference catchment only for soil type A (Fig. 4a). Indeed, in soil with high infiltration capacity, almost all the precipitation is infiltrated. In this case, excess of runoff from the corridor immediately upstream the outlet (with no other downstream under-panel areas and corridors available for infiltration) assumes a greater relative weight. However, the increment of peak flow and total runoff showed in Fig. 4a is not significant in absolute terms (peak flow for unit of width less than 0.25 L/s). The same conclusion can be drawn for other simulations with soil type A in which differences in the outflows from the solar park and from the reference catchment are observed.

Finally, Figs. 5a, 5b and 5c show outflows from solar parks of extension 2 and soil type C for constant precipitation input of durations 10 minutes, 1 hour and 3 hours, respectively, and 20-years RP.

For fixed solar park extension and soil type, outflow from the solar park is equal to that from the reference catchment (in terms of peak flow and total runoff volume) regardless of the duration of the precipitation event as shown in Fig. 5a, b and c.

Temporal distribution of the rainfall does not influence the aggravation of outflow from the solar park as compared to the reference catchment. Indeed, Figs. 3b and 7b report results of simulations carried out for parks of same extension and soil type, in which the 1-hour, 20-years RP precipitation event is given as input in the form of a Chicago hyetograph (Fig. 3b) or constant rainfall (Fig. 5b). In both cases, outflow curves from the solar park follow the corresponding curves from the reference catchment during the whole simulation.

Finally, simulations that differ only for the RP of the precipitation event also led to outflow curves from the solar park practically equal to those from the reference catchment, regardless of the magnitude of the event.

Globally, no significant increments of the peak flow and of the total runoff volume from the solar park as compared to the reference catchment were observed in all the 135 simulations for short-term condition. This result is in line with modelling and experimental findings of previous studies (Cook and McCuen 2013; Wang and Gao 2023).

To evaluate the effects of possible long-term changes in land cover induced by the presence of the PV installation, a reduction of the roughness surface is supposed for subcatchments representing corridors and under-panel areas in the model. In particular, the original Manning coefficient for those subcatchments is progressively reduced by 10% and 20% (i.e., n = 0.135 and n = 0.12, respectively). Figure 6 shows comparison between outflows from the PV solar park and from the reference catchment for 3 long-term condition simulations. Model parameters and precipitation inputs of the simulations related to Fig. 6a, b and c are the same of those discussed for Figs. 3b, 4b and 5a, respectively.

In all the simulations, the reduction of the surface roughness surface in corridors and under-panel areas leads to increased peaks flow as compared to the short-term analysis. For the events analysed in Fig. 6, a 10% reduction of the Manning coefficient results in peak flow increases of about 6% (Fig. 6a), 8% (Fig. 6b) and 21% (Fig. 6c). The percentage of peak flow increase is even greater if the Manning coefficient is reduced by 20% (15% increase for events in Figs. 6a and b and 35% for event in Fig. 6c).

The reduced roughness surface allows for a faster surface runoff from the upstream part of the solar park towards the outlet. Indeed, both the rising and the recession limb of the hydrograph from the solar park are anticipated with respect to those of the hydrograph from the reference catchment. Runoff velocity increases passing from a 10% to a 20% reduction of the Manning coefficient for all the events showed in Fig. 6. Also this result is in agreement with simulation studies carried out by Cook and McCuen 2013).

As a consequence of the increased runoff velocity, total runoff volumes from the solar park are greater than those from the reference catchment for all analysed events. Indeed, the potential of water infiltration is related to the ponding time above the subcatchment surface, which decreases as runoff velocity increases. For the events showed in Fig. 6, runoff volumes from the solar park increase with respect to the reference catchment in the order of 1–3% for a 10% Manning coefficient reduction and in the order of 2–5% if the Manning coefficient is reduced by 20%.

In this paper a modelling framework for the simulation of stormwater runoff in ground-mounted photovoltaic solar parks is proposed. EPA-SWMM software is used in a novel way to model all the elements in the solar park and their mutual interactions during precipitation events.

The modelling exercise showed the potentialities of the proposed conceptualization. Specifically, by comparing outflow discharges from the park and from a reference catchment (pre-installation condition), the proposed approach was successful in simulating some of the main impacts of PV power plant realization on peak flow and total runoff volume. In particular, simulations were run considering 3 different sizes of the PV installation (small, medium, large), 3 different soil types and input hyetographs of different return periods, shapes and durations. A first set of simulations have been run by assuming the same roughness surface for the solar park and the reference catchment (short-term condition). Then, the impacts of the presence of the panels rows (as well as of the maintenance activities) on the roughness surface have been considered. The proposed conceptualization allows to successfully simulate the spatial redistribution of the rainfall and infiltration fluxes due to the presence of the panel rows. The modelling exercise shows that when the surface roughness of the solar park is decreased, peak flow increases in the order of 6–35% as compared to the pre-installation scenario. Increased values (1–5%) of the total runoff volume are obtained as well.

The proposed modelling framework may be useful for operators in the field of photovoltaic for the evaluations of the outflow discharge from the solar park for different configurations of the installation, soil type and ground cover. The use of a free and open-source software adds value to the research and could represent a boost for the development and the improvement of the modelling framework as well as for a simple and wide diffusion of the results. Future availability of experimental data on runoff from solar park would help increasing the reliability of the obtained results; further investigations will also attempt to extend model applications at the watershed scale.

Future steps of the work may include the evaluation of other impacts on the ground cover induced by presence of the PV installation as well as the investigation of the solar park behaviour in long period simulations. Finally, other layouts of the PV installation should be considered in future works, especially those implying a reduction of the available area for infiltration within the park.