High retention is not the answer to drought


High retention is not the way to deal with drought – culverts, dams, artificial lakes, dry and wet retention. Other solutions must be resorted to.

The subject of water retention ‘springs up’ at almost every flood and during periods of drought. In the former case, we usually complain about an excess of water, in the latter case, about a serious shortage. But what exactly is water retention, much less large and small ones?

Water retention – what is it?

It is a series of interventions that enhance or regulate the ability to accumulate water resources and hold them for a longer period of time in the biotic (animate) and abiotic environment (usually anthropogenic areas – altered by humans, e.g. concrete reservoirs). In the practice of water resources management, water retention refers to the temporary retention of water in various forms in the environment, including rainwater, snow and ice, as well as groundwater, slowing its velocity and allowing it, in the case of natural retention, to seep into the ground. In many cases, retention focuses on significantly reducing the velocity of water flowing over the land surface, i.e. surface runoff, but also on reducing the velocity of water flowing along the bed of a river, stream or creek. This has the obvious effect of slowing down the water cycle in the environment and reducing the force with which flowing water erodes various elements in the natural and artificial landscape.

The main criterion for the division into high and low retention is the capacity of the reservoirs in which water is stored. In Poland, the limit value for the capacity of small water reservoirs is assumed to be 5 million m3. This size was defined in the Agreement of 21 December 1995 between the Minister of Agriculture and Food Management and the Minister of Environmental Protection, Natural Resources and Forestry concerning cooperation in the field of small retention. Reservoirs whose capacity exceeds 5 million m3 are counted among the solutions forming the so-called large retention. Unfortunately, very little is said in our country about solutions that can be described as natural large retention, focusing almost exclusively on practices related to damming rivers with large dams, creating artificial retention reservoirs (wet and dry) or land reclamation (mainly draining wetlands, regulating rivers).

Before going on to explain the term ‘large-scale natural retention’, let us further analyse what solutions are covered by the concepts of wet and dry retention. The concepts quoted refer to so-called reservoir retention. Let us imagine that we have a larger or smaller river into which water flows from a specific area (catchment area). Such runoff, and thus the volume of water, is highly variable in nature. Usually, after a snowy winter, during the snowmelt period and during summer heavy rainfall, there is a large volume of water that runs off the catchment in a very short time. This state of affairs, is often the cause of localised flooding or major floods and is due to the fact that the river bed is unable to drain such a large volume of water. The excess of this water occurs from the riverbed flooding riverine areas where the natural landscape (e.g. floodplain terraces) favours this. Such extremes are repeated almost every year. Droughts are another type of extreme. These occur when no precipitation is experienced for an extended period of time and high air temperatures contribute to intense evaporation and consequently lead to water deficits (water begins to run out in the soil, then in surface reservoirs and watercourses, and even in groundwater bodies). All too often, both of these extremes are attempted to be solved by the construction of water reservoirs, which are designed to capture water in the case of excess rainfall and, in the case of drought, to slowly release the stored water into the environment. This area is again divided into dry and wet reservoirs. A dry reservoir is one that is only periodically filled with water (in the event of a flood). How quickly it is emptied depends on the technical design and its location. Very often, the water stored in such a reservoir is quickly pumped out in order to again prepare such a reservoir to receive the next excess water and to ease the flood wave. Unfortunately, when a drought occurs, no water is stored in such reservoirs, which can then be used to replenish the environment. However, such possibilities are offered by wet reservoirs, in which excess water is stored for a much longer time, which makes it possible to use it in periods of drought, e.g. for agricultural purposes, to irrigate fields. Regardless of the type of reservoir retention chosen (dry or wet), drought and flood prevention in the case of artificial reservoirs applies only to those areas in the immediate vicinity of rivers and the reservoirs themselves. The range of impact of such solutions is only local, and the investment and environmental costs associated with the construction of such reservoirs are very high (the need to divide rivers, build costly passageways allowing the migration of various fish species, public consultations and even expropriation). However, focusing solely on reservoir retention will not solve the problem of flooding or drought in areas further away from rivers and lakes, artificial reservoirs. The effective solution in these areas should be large-scale natural retention in the soil and in marsh and wetland ecosystems. This is by far the most effective solution, and one that can be implemented throughout the country, with far less unit costs and almost zero environmental and social costs.

Large-scale natural retention can be implemented both in cities and in agricultural and forested areas. It is enough to gradually increase the content of organic matter in the soil, introduce mid-field afforestation and bushes, agroforestry and regenerative agriculture solutions, and in towns and cities to stop the rampant “concretosis” (i.e. the tendency to cover large areas with surfaces impermeable to water – concrete, asphalt and buildings).

Given that more than half of Poland’s land area is occupied by agricultural areas, our greatest hopes can be pinned on the introduction of best available practices and best available solutions for large-scale natural retention, precisely in the agricultural sector. All hope lies beneath our feet – in the soil. Soil is a remarkable medium which, when the soil is of good quality, has an extraordinary capacity to retain and store water. It all depends on the percentage of organic matter in the soil. The organic matter (i.e. humus, humus substances) influences the formation of the soil’s tuberous structure, which translates into improved water-air relations in the soil. In simple terms, the higher the humus content of the soil, the more water and air there is in such a soil. In the context of our considerations, we are interested in the ability of humus soil to store water. 1 gram of soil humus is able to bind up to 8 grams of water in its porous (spongy) structure! The humus content is a basic indicator of the fertility of any soil. Humus is a complex, amorphous substance made up of a variety of organic and mineral-organic substances. The sources of these substances in the soil are the various decomposition processes of organic remains (plant and animal) carried out by soil micro- and macro-organisms (bacteria, protozoa, fungi, invertebrates such as earthworms). Together with calcium, magnesium and mucilage produced by bacteria and fungi, humus compounds have the ability to bind soil particles together in larger soil aggregates, contributing to the formation of the aforementioned tuberous structure. In addition, they are a natural, organic fertiliser providing plants with essential nutrients, vitamins and trace elements. The more humus in agricultural soils, the better the growth of yields, but when agroforestry solutions are introduced, also of trees and shrubs. More trees and shrubs mean more shade and a greater regulating influence on local water relations. More shade means lower temperatures on hot days and more stable conditions on cold and windy days – this is also of great importance in terms of high natural retention.

The humus content of soils in Poland varies greatly. It ranges between 0.6 and 6%. Humus-poor soils (i.e. soils with less than 2% humus content) account for 56% of all soils in our country on average. In addition, looking at trends over the past decades, there has been a significant decline in the humus content of agricultural soils, with a consequent drastic reduction in the ability of soils to retain water, mitigating the effects of floods and increasingly severe droughts. Can this process be reversed? Due to its unique nature and physico-chemical properties, humus increases the buffering capacity of soils and may be the best tool to address the resilience of our areas to both excess and scarcity of water. The humus particles have a highly developed surface area – 1 gram is about 1 000 m2. In comparison, 1 gram of sandy mineral soil without humus is only 3-60 m2. Particle size also has a strong influence on water storage capacity – the smaller the particle, the greater the water retention capacity. The humus particles are on the order of 0.001-0.003 mm in size, while the size of sand grains is 1-0.1 mm. Thus, humus particles are a thousand times smaller than sand grains and can therefore retain enormous amounts of water, up to five times their weight. Analyses of soil samples taken from agriculturally used land show that at a content of 1% per hectare of land, the weight of humus is 30 tonnes. This means that this amount of humus per 1 ha absorbs as much as 150 tonnes (approximately 150 m3) of water.

Among the good practices associated with high natural retention should be the explicit practice of increasing soil humus content. This can be done in many ways and not only in areas with an agricultural character. In cities, we should pay particular attention to the issue of separate waste collection and the recovery of organic waste to produce compost, which can then be added superficially to the soil by spreading it in a thin layer on every urban lawn, around trees and shrubs, in parks and green spaces. Within urban water retention, it is also worth highlighting the role of urban landscaping practices. It is a common bad practice to design paved surfaces lower than the green areas running alongside them. Please pay attention to this on your next walk. Is the pavement you are walking on lower than the grass running alongside? Is the road surface lower than the roadside green belts? All paved surfaces, impermeable to water, during heavy rainfall create the beds of periodic rivers that flood our cities. However, if all paved and sealed surfaces were designed to be at least 10 cm higher than lanes and green areas (whose soils are additionally enriched with compost), we would have another tool to support rainwater management in urban areas.

In addition, I encourage every urban resident to set up neighbourhood and community compost piles. Start learning proper methods for composting your own organic waste and make compost and spread it on the lawns where you live. It’s time to take matters into your own hands! Those living in agricultural areas have an even greater repertoire of techniques to significantly increase water retention in these areas. Take care of the earthworm populations in your fields, meadows and pastures. On 1 ha of well-managed field, enough earthworms can live to enrich the soil in one year with 15 tonnes of vermicompost, which they produce for free, locally. That’s another 75 tonnes of water bound in the soil. All we need to do is reduce the frequency of ploughing or switch to no-till systems, introduce pre-crops, intercrops and catch crops that provide organic matter to the soil, and increase investment in mulching soils during their periods of dormancy (e.g. with straw). Let’s also start to introduce agroforestry practices on a large scale (strip cropping between rows of trees and shrubs, planting on baulks, mid-field remixes, forest-pasture systems, etc.). The presence of trees and shrubs makes it possible to reduce the negative effects of violent weather phenomena, which we face in the face of increasing climate change. Also, the effects of floods and droughts are less severe in wooded areas, and the presence of shrub and woody plants further stabilises soils and reduces their susceptibility to water and wind erosion.

These and many other solutions aimed at increasing the water retention potential of soils should receive the greatest emphasis in the coming decade. Building more large reservoirs will change little. Building humus content in the soil will change everything.

Author: Łukasz Nowacki

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