Microretention – an activity for everyone, but the devil is in the detail


The premise of the MIKRORETENTION programme, designed to support small-scale retention measures, speaks of facilities with a volume of tens of thousands of m³ and an area of up to 1 ha. For a larger catchment area, these volumes are negligible, but in a source area or a small catchment area of a top-order tributary, such reservoirs can do a lot – both good and bad. The devil, as usual, is in the details of implementation. Before a bit more about the idea, some basic thoughts about water and landscape. Every Pole probably knows that there is not much water available to us in our country. As I write these words (28.06.2021 – FAF’s footnote), more than 40 municipalities – from Podlasie to Lubuskie and from Rzeszów to the suburbs of Szczecin – have announced appeals to save water and limit its use to the necessary minimum[1].

Every year, a significant part of our country’s territory is affected by drought. In recent years, this condition has started slightly earlier and lasted longer. A hint of a snowy winter and a cool spring have apparently made little difference – a few days of hot weather and the drought comes to the fore. Every now and then, more flooding is added to it, and localised sudden flooding is now the norm, often at the same time and with no impact on the drought condition. The extent and severity of drought is increasing, especially in the last two decades, and it is no longer only agriculture that is a victim. Forests are drying up, rivers are drying up, lake levels are dropping. Why is this happening, and is climate change the cause of these uncomfortable situations? Let’s look at the general and historical data and the conditions that determined the initial state we had. The surface water resources in our country come 100% from rainfall. In our territory, we do not have mountains high enough to sustain glaciers feeding our resources. Moreover, the future of the latter and the systems dependent on such a supply is now uncertain across the Planet. How much of what falls will become a resource available to us – the inhabitants – depends to a large extent on the state of the landscape, both on the inanimate and animate side of it[2]. The relief and geological structure are an important factor in the division between the part of the precipitation that evaporates, soaks in and drains down the geosystem, according to the law of gravity. Nature has arranged the balance over thousands of years, in the rhythm of which a mosaic of habitats has emerged in our part of Europe. The composition of this natural mosaic is due to the amount of water available in our landscape – through rainfall[3]. More than half of the precipitation evaporates and there has been a noticeable trend towards an increase in this proportion in recent years. This is due to climatic conditions and the transformation of the landscape morphology: changes in the land cover structure – from permanently green areas to crops (about 60% of the area), or buildings/sealing (about 10% of the area). Forested areas – about 30% of the area[4] – are relatively stable, although no longer natural.

Given that we have quite little influence on the evaporation process itself, limiting the escape of water from our territory is mainly possible with regard to the run-off part. Non-evaporated precipitation – rain and snow (after melting) – partly soaks into the soils and partly runs off the surface. What is important for ensuring the water supply of all components of the environment – from people, to all the rest of animate nature, to the recharge of watercourses and standing waters – is the proportion of water that soaks in to that that flows over the surface.

The second equally important aspect of this inevitable migration of water – in accordance with gravity – towards the lowest areas and the sea, is the speed at which this run-off takes place. The principle is simple. The more impervious the surface, the more water runs down it at a rate limited only by friction at the interface. The difference in the rate of surface run-off compared to underground run-off is on average 1:20. This means that the more water that remains in the soil, the more will slowly feed our lakes, rivers and underground resources from which, among other things, we draw our drinking water. Land cover also has a significant impact. The run-off rate of water flowing down a thin layer in a vegetated river valley, compared to a paved surface, is several times lower. What this means is that it is not beneficial to regulate watercourses in order to reduce flooding on the floodplains of the valleys, starting from their very sources, as this accelerates water run-off. This is detrimental to water resources. It is beneficial to re-establish modelled natural, varied and meandering watercourses that spill over wherever acceptable. In this way, we slow down runoff – promote retention – while reducing the threat of both drought and flooding.

It is possible to take action to slow the run-off of water, or even to retain it, called retention. The landscape is naturally capable of retaining quantities of water to ensure stable surface water recharge and groundwater replenishment at our latitude. Unfortunately, this potential has been enormously reduced over the centuries, and especially in recent decades, by land reclamation measures, changes to the landscape, its cover and its use. As a result of these ill-considered measures, today we have a serious problem with the increasing threat of drought.

Depending on where we are in the catchment, we can divide remedial action with regard to water run-off into low and high retention, although these are still very conventional terms. The key is the restoration of natural retention, whose potential in relation to any kind of artificial reservoir is incomparably greater. While in artificial measures we are talking about constructed volumes counted in millions of cubic metres, in the natural retention of a small river – with a catchment area of around 2-3 thousand km² – we are dealing with a soil capacity of the order of 0.5 km³!!! Of course, the magic of big numbers works on laymen, but the truth is that it is eminently more effective to take even small but multiple measures to help improve retention as far away from the river as possible. Such actions can be carried out by the farmer, the local authority and even the city. Hence, the currently proposed programme for improving rainwater retention called micro-retention, as an action “for everyone”, is fully justified. The importance of individual measures of this type cannot, of course, be great, but economies of scale play a role in this case, and even a micro measure is important[5] – with sufficient repetition in a given catchment area, it can produce effects far more beneficial than the large investments still being pushed for today. Compared to them, micro-retention has the advantage of being much closer to the factual basis of modern water management, i.e. retaining water at the beginning of its circulation in the system, at the sources of the watercourses, rather than this circulation end, in the main river.

However, it should be noted that it makes little sense to define an action as micro, without specifying the scale to which we are referring.

In the programme we find many proposals to build or restore ponds, lagoons as facilities to catch water practically where it has fallen. Seemingly such an idea makes sense, however:

Depending on the proportion of change in evaporation, the watercourse on which such a reservoir will be built may turn into an intermittent one and instead of improving water relations, they will deteriorate below the reservoir or lagoon. It is much more beneficial to restore the highest possible bed levels in existing drainage systems, restoring and preserving wetlands, including both typical wetlands and wet meadows, which are still unnecessarily drained today.
We also read in the programme about digging ponds or lagoons. Here we already have a complete lack of understanding of how water functions in the landscape on the part of the authors of this idea. Every hole, ditch or similar interference in the landscape is a drain, so there can be no question of any improvement. An exception is a sealed reservoir or cistern, although even here there is an adverse interference with the morphology of the area. If retention is to be done, it should be done in a way that does not deteriorate water relations, in accordance with the articles of the Water Law[6]. It is necessary to propose methods to potential beneficiaries that are in line with scientific knowledge so as not to promote harmful solutions.

It is worth paying special attention here to the National Programme for Surface Water Restoration (NPRWP)[7], in which a number of measures can be found that directly fit the assumptions described in “Microretention”.

In the programme it would be worthwhile to explicitly separate the forest package. It should include not only the signalled restoration of technical facilities, with emphasis on meeting the requirements of the Water Framework Directive, especially ensuring morphological connectivity, or the reconstruction of tree stands. Crucial in the case of forests is the possibility to exclude forest areas important for the protection of water resources. There is a need to create water-protective forests, following the example of many countries that have long faced water scarcity problems. Forests of this nature are an excellent tool to support not only the protection of the wetlands themselves, but also to buffer the undesirable effects of anthropogenic pressure.

Alongside this, it is advisable to revise the approach to the landscape as a particularly important element of retention in each of its parts – agricultural, forest and urban: there is much to improve, such as the selection of certain maintenance measures according to the seasons, or their subordination to the principle of water conservation. The mowing of green areas or the felling of trees can serve as examples. In both cases, a simple analysis of ecosystem services – including water retention – indicates that both activities need to be changed. There is a place for these changes in the micro-retention programme – they serve to adapt the area to higher retention.

Author: Artur Furdyna

The text was written as part of the project “Hydrozagadka – how to win against drought?”.

[1] https://www.google.com/maps/d/u/0/viewer?fbclid=IwAR1I_7iVu-c-nS7uWL7f2kGR2msITbWabFsOBCKCDWubr0ebyFFyeAGPIYE&mid=1wEJUpVpQbWri3qGbKlPjZuYYSE8QkSoQ&ll=52.16024614178935%2C19.138021599999984&z=6

[2] https://www.senat.gov.pl/gfx/senat/userfiles/_public/k9/komisje/2019/krrw/materialy/135pos_3mgmizs.pdf

[3] https://koalicjazywaziemia.pl/wp-content/uploads/2020/11/Ekspertyza_Woda-w-rolnictwie.pdf

[4] https://land.copernicus.eu/pan-european/corine-land-cover/clc2018


[6] Section 234 of the Water Act , https://isap.sejm.gov.pl/isap.nsf/download.xsp/WDU20170001566/U/D20171566Lj.pdf

[7] https://www.wody.gov.pl/images/Aktualnosci/foto/renaturyzacjaKPRWP/Podrecznik_renaturyzacji.pdf