Show lessMeine van Noordwijk, Pieter van Oel, Catherine Muthuri, Usha Satnarain, Rika Ratna Sari, Paulina Rosero, Margaret Githinji, Lisa Tanika, Lisa Best, Gildas Geraud Comlan Assogba, George Kimbowa, Federico Andreotti, Elisabeth Lagneaux, Charles Nduhiu Wamucii, Arief Lukman Hakim, Andrew Miccolis, Ali Yansyah Abdurrahim, Ai Farida, Erika Speelman, Gert Jan Hofstede

First Published January 27, 2022 

Research Article https://doi.org/10.1177/00307270211073813

Abstract

Metrics of hydrological mimicry (‘mimetrics’) reflect similarity in ecological structure and/or functions between managed and natural ecosystems. Only the land-surface parts of hydrological cycles are directly visible and represented in local knowledge and water-related legislation. Human impacts on water cycles (HIWC) can, beyond climate change, arise through effects on local and regional hydrological processes, from both reduced and increased water use compared to a natural reference vegetation with which landscape structure and hydrology are aligned. Precipitationsheds, the oceanic and terrestrial origin of rainfall, depend on evapotranspiration and thus on vegetation. The political commitment to reduce agricultural impact on nature requires hydrological mimetrics to trickle down through institutions to actions. Existing metrics do not suffice. For example, the water footprint metric that relates agricultural water use to consumption decisions, suggests minimizing water use is best, ignoring full hydrological impacts. We explore principles, criteria and indicators for understanding HIWC, via modified evapotranspiration, effects on streamflow (downstream impacts) and atmospheric fluxes and precipitation (downwind impacts). Comprehensive HIWC mimetrics for a set of pantropical watersheds suggest hydrological mimicry options for forest-derived land use patterns through intermediate densities of trees with diversity in rooting depth and water use, interacting with soils, crops and livestock.Keywords blue water, evapotranspiration, green water, nature-based solutions, rainbow water, water footprint, watershed functions

Introduction

Maintaining hydrological functions and cycles is essential for meeting the sustainable development goals at the intersection of water, energy, food and income (van Noordwijk et al., 2018). Increased water demand and overexploitation of limited freshwater resources leads to water scarcity, and in many places around the world, to conflicts over water. Yet, current human domination of the global water cycle beyond water allocation issues is absent from widely used depictions and perceptions (Abbot et al., 2019a). Water cycles connect vegetation, soils, landscapes, geomorphology, and climate to human appropriation of resources, which is driven by a growing population, consumption patterns and lifestyles (Mekonnen and Hoekstra, 2011; Flörke et al., 2018; Hoekstra, 2019). Water scarcity implies that the combination of timing, place, and water quality on the planet does not match human demand, while the global amount of available renewable freshwater may yet be adequate. Rather than as a hydrologic constraint, ‘agricultural economic water scarcity’ has been defined as a lack of irrigation due to limited institutional and economic capacity to get the right quality of water to the desired places at the right time (Rosa et al., 2020). However, beyond engineering costly canals, reservoirs and pipes (Gohari et al., 2013) to overcome hydrologic constraints, solutions are needed to reduce the ongoing human impact on the water cycle (HIWC). Water availability in large parts of the tropics depends on ‘water towers’ (defined in Dewi et al. (2017) as humid upper watersheds with drier downstream areas) generating river flow and through the atmosphere (as ‘flying rivers’, streaming atmospheric moisture; Schwarzer, 2021). Caps to the monthly river water flows that can be allocated to human uses are needed to ensure that water appropriation for human uses remains within ecological boundaries, earmarking sufficient water for aquatic subsystems (Hogeboom et al., 2020). However, river flows (‘blue water’) represent only about one-third of total rainfall over land, with the other two-thirds used in situ by direct evaporation and transpiration by vegetation (‘green water’; Stoy et al. 2019). The continued availability of atmospheric water vapour as the source of precipitation (‘rainbow water’) cannot be taken for granted (van Noordwijk et al., 2014a; Creed and van Noordwijk, 2018). Beyond issues of allocation of river water, the full hydrological cycle and its functions needs to be first understood and then managed (Abbott et al., 2019b). Additional metrics are needed (van Noordwijk et al., 2016).

The classification Meadows (1999) developed of ways to intervene in a complex adaptive social-ecological system can be of use here. It ranks parameters (‘system-state data’), feedbacks (relationships, impacts), institutions (rules of and roles in the game) and goals as having an increasingly ‘transformative’ impact on a social-ecological system. The most profound changes are at goal level: beyond farmer income and societal food supply, reducing negative impacts on biodiversity and climate are goals for agricultural land use, since they were accepted as high-level sustainable development goals. To trickle down through institutions to actions, political commitments to reduce the impacts of agricultural land and water use on nature will require appropriate metrics of hydrological impacts, followed by norms, standards, regulation, incentives, monitoring and evaluation. As these metrics for minimizing negative feedback can reflect a degree of ‘mimicry’ of nature, we propose to call them ‘mimetrics’. They can derive from ‘mimesis’ that explores the relationship between an image (in this case ‘agriculture’) and its purported original form (in this case ‘nature’), go beyond imitation, and seek effective ‘memes’ that can lead to cultural reproduction (Potolsky, 2006). In the biological tradition the term mimicry is associated with, often ‘superficial’, resemblance that leads to mistaken identities and reduced predation pressure. Current ecosystem mimic concepts derive their relevance at the ‘feedbacks/relationships’ level from the experience that deviation from the functioning of pre-human ecosystems can lead to far-reaching ecological and hydrological disturbances that may be avoidable (van Noordwijk and Ong, 1999). As common biological examples of mimicry refer to the colour patterns of animals, hydrological mimicry and the functioning of hydrological cycles can be explored through the ‘colours of water’ (blue, green, grey, rainbow).

In the context of this Special Issue on biomimicry and nature-based solutions for agricultural issues of concern, we set out to explore the contributions various metrics can make to support public awareness of human influence on the hydrological cycle and the ways it can be reduced. Our specific questions for this ‘perspective’ piece are:

1. How are human impacts on water cycles related to the types (colours) of water across spatial and temporal scales?
2. What has been the primary appeal and use of existing mimetrics?
3. What metric could best represent human impact on water cycles across spatial and/or temporal scales?
4. How do proposed mimetrics relate to the diversity of issues identified in a pantropical set of forest-water-people landscapes?

We reviewed the literature and identified relevant current debates, without claiming the comprehensiveness of a formal systematic review of the literature.