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Mechanical Screening due to pore size Feed in particulate matter Immobilization of particulate micro-pollutants Physical Surface area to adsorb dissolved compounds Supply in dissolved compounds (pollutants and nutrients) Adsorption of dissolved pollutants Physical-Chemical Chemical reactivity of geological background Supply in TEA, pH buffering and reactive chemical elements Precipitation and physical-chemical redox processes Biological Physical support for microbiocenosis Supply in living microorganisms Biodegradation, assimilation and self-purification

      1.16.2 Ecohydrology on Small Watersheds

Ecohydrology suggests addressing water resource management at the medium-scale catchment areas (up to 300 km²), where the origin of the harmful effects of anthropogenic activities (urbanization and agriculture) can be detected. It is then possible to balance the use of ecosystem services with their natural renewal. Under natural conditions (Figure 1.14a), biodegradation and bio-transformation of organic substances carried by water flows achieve a dynamic equilibrium, which is a factor in the resilience of the system (Holling, 1973). In an urbanized environment, the geomorphological alterations of rivers and banks limit the biodegradation capacity. In addition to these physical alterations, there is also the contribution of point source pollution, via urban discharges in rainy weather. The result is an imbalance that is the source of a disturbed situation. In order to regain a balance (Figure 1.14b), it is necessary to understand which variables need to be tuned and how to tune them. For the biocenosis part, two characteristics are to be considered: the first one is the kinetics of natural regeneration process. Humans within a certain limit can modify it. The second one is the quantity of biotransformed matter. This quantity depends on the surface or volume of the support medium. We find here the principle of the flux equation: flux equals the product of the process kinetics by the reactional volume. It is therefore possible to increase a flux by acting for example on the reaction volume (Figure 1.14b). Here, we see the variables of a spatialized management of ecosystem services in a watershed. It is thus possible to modify land uses to favour certain processes. It is also possible to amplify certain biotransformation processes, such as biodegradation, by developing ecological engineering based on NBSs.

Figure 1.14 (a) illustration of balance and imbalance nutrient and biodegradation flux; (b) Illustration of the use of flow control variables.

      Ecohydrology approach allows, for example, the design of CWs because the knowledge of the dynamics of the processes is defined (Lee et al., 2009). Without wanting to turn water bodies into treatment plants, it is necessary to help them eliminate the pollutants that contaminate them. Bioremediation techniques propose to stimulate natural processes in order to help water bodies to actually or only apparently remove pollutants (they are always present, but hidden).

      However, a third characteristic is to be considered in the spatial management of water flows and associated energies: it is the water path in the different compartments of a watershed. It is useful to have at this stage some pieces of information on the transfer velocities of water and associated substances, both dissolved and particulate, in a watershed. This will depend on the watershed topography, the key determinant of surface runoff and flows, its soil nature which influences the infiltration rate of water into the soil, the land uses which will modify the chemical composition of the water, and the geology which will influence the link between long travel time groundwater and short travel time runoff. It is also necessary to mention the presence of natural bio-catalysts such as bacteria present in the soil, the interfaces of transitions (ecotones) between terrestrial and aquatic environments such as ripisylves and the hyporheic zone (Krause et al., 2017). These multiple interactions give rise to “hot spots” of metabolism at the interface of terrestrial and aquatic environments, in certain places and at certain “hot moments” (McClain et al., 2003).

      Wetlands and temporary streams are a good illustration of the latter. A good knowledge of water paths is therefore necessary to identify places with a high potential for natural metabolism in the catchment area. Water paths on magmatic or metamorphic formation will generally present essentially surface flows with a low quantity of underground water. Land use management along the waterways will then have a preponderant effect on the quality and quantity of water related to rainfall.

      Because the control factors involved in these runoff stages are of different natures and spatial resolution, their respective roles are assessed on a qualitative scale (Lagadec et al., 2016). A scoring method is then used to calculate at which locations in the catchment area the factors contribute most to creating situations with high metabolic potential. The resulting maps are only valid if they are validated by ground truths according to a described methodology (Braud et al., 2020). Intense runoff mappings correspond to heavy rainfall events that are responsible for significant movement of particulate and dissolved matter with runoff and hypodermic flows. We can speak of “hot moment at hot spot”. In comparison, underground flows are slow and contribute above all to the dissolution of matter over time. These dissolved elements are remobilized by exchanges between surface runoff and hypodermic flows where oxidation and reduction reactions are driven by microbiological activity, variations in water content and soil temperature. The identification of metabolism hot spots indicates where in the catchment area the implementation of water and energy flow management strategies would be most effective.

Figure 1.15 A pluvial runoff model can help identify which part of the basin is more prone to accumulate water; potential humid zone can be identified.

This strategy can be illustrated (see Figure 1.15) at a catchment scale using the Indicators of Pluvial Intense runoff (IPIR) method (Dehotin et al., 2015).

      Runoff is represented by three stages occurring in sequence and be repeated along the slopes before reaching concave topographic breaks, creating local wetlands or contributing to streams flows.

      1 1) The genesis stage, which expresses the potential of an area of the watershed to produce runoff. It can ranked from zero to very high potential;

      2 2) The transfer stage, which indicates flow paths with erosive potential;

      3 3) The accumulation stage, which indicates areas where water and solid deposits concentrate, accumulate and settle on slopes and watercourses.

      The example treated is located in the peri-urban area of Lyon. Comparison with land uses allows the construction

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