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Integrating blue and green water flows for water resources management and planning
Physics and Chemistry of the Earth 31 (2006) 753–762 www.elsevier.com/locate/pce
Integrating blue and green water flows for water resources management and planning Graham Jewitt
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School of Bioresources Engineering and Environmental Hydrology, University of KwaZulu-Natal, South Africa
Abstract The ‘‘Green Water’’ approach, where flows of water vapour in the form of transpiration, interception and evaporation from the soil and vegetation is considered green water and runoff and groundwater recharge is considered blue water, has been an extremely useful illustrative concept in many situations where the role of land use in water resources management needs to be highlighted. The approach has been the subject of much interest in recent years, particularly in semi-arid and arid regions where Green Water Flows dominate the hydrological cycle. However, it is clear that there are limits to the concept in informing water resources management and planning. In this paper, these limits are explored through case studies of commercial afforestation and runoff harvesting in the SADC region. Issues highlighted include the degree of simplification of the hydrological cycle in many green water focused studies, appropriate spatial and temporal scales for the consideration of low flows and the uncertainty regarding the storage of water in the soil profile and the generation of flows from saturated and unsaturated soil water. It is concluded that rather than focusing on green or blue water flows, it is the hydrological linkages between these and their representation in water resources management and planning that needs most attention. 2006 Elsevier Ltd. All rights reserved. Keywords: Green Water Flows; Integrated water resources management; Afforestation; Rainwater harvesting
1. Introduction The ‘‘Green Water’’ approach has gained prominence since it was initially introduced by Falkenmark in 1995 (Falkenmark, 1995). The promotion of Green Water Flows has been extremely effective in highlighting the role of total evaporation (E) from the landscape in the hydrological cycle and the potential to utilize the transpiration portion of these flows for food and fibre production. However, although the concept has been extremely useful in this regard, the consideration of Green Water Flows in formal water resources management and planning is proving to be extremely difficult. Green Water Flows have been the subject of much interest in recent years, particularly in semi-arid and arid
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1474-7065/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pce.2006.08.033
regions where they dominate the hydrological cycle. In these areas, small changes in Green Water Flows can result in major impacts on downstream blue water flows. As highlighted, by Falkenmark and Ro¨ckstrom (2005), this provides a major dilemma in that meeting food requirements for future populations will require additional utilization of land and therefore, water resources and that conflict with other users, in particular users who are reliant on run of river abstractions, is inevitable. These users are typically the most vulnerable, e.g., rural communities and aquatic ecosystems. Historically and inevitably water resources planning management is focused on blue water. If Green Water Flows are to be considered explicitly in future water resources management, it is imperative that the land segment i.e. the catchment is more explicitly considered in water resources management. This cuts to the heart of some of the debate of the mid-1990s which considered Integrated Catchment Management vs. Integrated Water Resources Management (IWRM). In essence, much
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of the debate was about the extent to which management of the catchment i.e. the landscape could be included in approaches which claimed to provide for Integrated Water Resources Management. In South Africa, it is recognized that the ultimate goal is Integrated Catchment Management in which all water users, including non-irrigated land use are considered, but it was accepted by DWAF and WRC (1998) that IWRM in South Africa’s immediate future would ‘‘comprise mutual and sensitive dependence of water, land-use and aquatic ecology management, but with incomplete integration of natural resource management’’. Consequently true Integrated Catchment Management remains a very distant goal, although IWRM continues with a catchments or watersheds as a fundamental management unit (Jewitt, 2002a). Increasingly, scientists are realizing that the consideration of Green Water Flows in water resources management will require a higher level of integration which explicitly considers land issues together with water issues, sometimes termed ‘‘Integrated Land and Water Management’’ (Berry et al., in press) or ‘‘Integrated Land and Water Resources Management’’ (ILWRM) (Falkenmark and Ro¨ckstrom, 2004). It has been suggested that ILWRM may require policies that are more explicitly directed at the management of the land resource as it affects evaporation and water resources – i.e. Green Water Policy Instruments (Calder, 2005). But what form will such instruments take, and do any exist? Whilst it can be argued that Green Water Flows have not been adequately considered, water resources management and planning must consider the potential impacts on blue water flows. The hydrological difficulties pertaining to the consideration of Green Water Flows in policy arise from two issues: • The difficulties in measurement or estimation of wateruse of a land based activity. • Understanding and predicting the links between Green and Blue water flows and the spatial and temporal responses of catchments. Whilst from a water resources management and planning perspective, the primary issues arise from: • Difficulties in managing the landscape within water resources management legislation which is inevitably focused on blue water. Thus, these three issues are highlighted in this paper. 2. Quantifying green water flows Most often Green Water Flows are considered to represent Total Evaporation, (often called ‘‘evapotranspiration’’) and are considered to comprise both evaporation and transpiration components. The evaporation component is made up of evaporation of intercepted water evap-
oration from plant surfaces as well as free water surfaces, and evaporation from the soil. This component is considered non-productive and in order to distinguish it from transpiration, it is sometimes been referred to as ‘‘white water’’ (Savenije, 2004) highlighting the concern that, hydrologically, it is problematic to lump these two components. Although they interlinked, the processes governing their generation are different and despite the attractiveness and simplicity of the term ‘‘Green Water Flows’’, failure to highlight the different components making up E may compromise hydrological modelling efforts where, it is argued in this paper, good conceptual understanding is prerequisite for integrating blue and green water flows for water resources management. However, in order not to weaken the fundamental message of the green–blue approach the original and most commonly applied terminology i.e., where these are considered productive and non-productive Green Water Flows (Falkenmark and Ro¨ckstrom, 2004), is applied here with the clear recognition that these need to be considered separately in hydrological process studies. Indeed, it is the movement of water through agricultural crops and timber by transpiration (i.e. productive water use) that must be optimized if future food and fibre requirements are to be met, hence the evolution of the phrase ‘‘more crop per drop’’. There are several challenges associated with estimating or measuring Green Water Flows. In the first instance, difficulties in determining the spatial and temporal variation of E over large areas are compounded by the many factors that influence its occurrence and prevalence. These include local micro-climatic effects as well as different vegetation and soil types. The conventional approaches for quantifying E have been based on localized point measurements and do not allow for its estimation over large geographical areas. These approaches include direct measurements (evaporation pans, lysimeters, etc.), climatic stations (eddy covariance, Bowen ratio, etc.), heat pulse systems (transpiration only) and hydrological models (water balance and biomass production). Secondly, there are very few methods which allow for estimates of transpiration and evaporation separately. The Heat Pulse method is very useful in woody vegetation (e.g., Dye and Olbrich, 1993) but has proven problematic in herbaceous plants. Many Soil-VegetationAtmosphere models exist and usually, the total evaporation process is represented by some derivative of the Penman equation to separate evaporation into its components of interception, soil evaporation and transpiration. Notwithstanding this recognition, it is argued that effective and accurate estimation of Green Water Flows at large spatial scales as opposed to the more ‘‘traditional’’ point scale measurements will be highly beneficial to scientists as well as water resources managers and planners. They will provide both direct estimates of E as well providing data which can be used for the improvement of existing, and development of new models. In this regard, two of
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the most of the promising methods are the application of the scintillation method and remote sensing for estimates of E using the surface energy balance. 2.1. Estimation of Total Evaporation by scintillation Scintillation is a general term used to describe the changes in brightness of an object when viewed through the atmosphere. As an electromagnetic wave moves through the atmosphere it will be distorted by refraction, absorption and diffraction resulting in intensity fluctuations caused by heat, moisture and pressure variations. The Large Aperture Scintillometer is an instrument used to measure such fluctuations and operates at scales from 500 to 5000 m. The signal is transmitted by a light source and received by a receiver across a transect selected by the operator (Fig. 1). Additional data on temperature, pressure and humidity are necessary to compute sensible heat flux and ultimately estimate evaporation. Details of the method are provided by Kongo and Jewitt (2006). An important feature of the scintillation technique is that, although the measurement is along a path of a light beam, this actually provides an estimate of evaporation over an area because of the wide fetch (Meijninger and Bruin, 2000). In South Africa, three such instruments exist and are increasingly being used to estimate Green Water Flows from a variety of land uses including Sugar Cane (Wiles et al., 2005), Indigenous Forest (Dye, P. Council for Scientific and Industrial Research, Personal Communication, 2005) and rehabilitated mine dumps (Jarmain C. Council for Scientific and Industrial Research, Personal Communication, 2005). 2.2. Remote sensing of evaporation Remote sensing data provided by satellites provides a means of obtaining consistent and frequent observations
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of spectral reflectance and emmitance of radiation from the land surface at scales ranging from 30 m and larger. Various methods have been derived in order to estimate E from these spectral radiances, probably the most well known of which is the Surface Energy Balance Algorith (SEBAL) (Bastiaanssen et al., 1998a). SEBAL is an energy partitioning algorithm comprised of 25 computational submodels that calculates E and other energy exchanges at the earth’s surface (Bastiaanssen et al., 1998a,b). The algorithm computes most essential hydro-meteorological parameters and does require limited ground based meteorological data. It has been applied recently in the Thukela Catchment (Kongo and Jewitt, 2006) and the Limpopo Basin (Ahmad et al., 2005). Based on available measured E, initial results obtained seem reasonable, but also of importance is the spatial variability in Green Water Flows which can be derived from the method as illustrated in Fig. 2. One of the intermediate outputs of the SEBAL algorithm is the sensible heat flux for each pixel of the satellite image used. This provides an opportunity to validate SEBAL output using measured sensible heat flux data obtained from a Large Aperture Scintillometer as described above. 3. Impacts of land use change and green water flows The consideration of the land use change in water resources management and planning is problematic. However, in South Africa, because of its high levels of green water use and resulting impact on downstream blue water resources, the establishment of commercial afforestation has been controlled for over 30 years. Thus, the potential for impacts on the water resource resulting from a change in land use are well known and consequently the enthusiasm to embrace Rainwater Harvesting (RWH) approaches is tempered with some concern regarding whether there are likely to be downstream impacts on blue water flows. 3.1. Streamflow reduction activities (SFRA): a step towards green water policy?
Fig. 1. A Large Aperture Scintillometer in operation above a sugar cane transect.
In South Africa, forestry has historically been recognized as an important water user and concerns for the protection of water resources have led to the control of commercial afforestation and forestry practices since 1972. Under the National Water Act (NWA) (Act no. 36 of 1998) forestry is considered a ‘‘stream flow reduction activity’’ (SFRA) and as such must be licensed as a water user. A stream flow reduction activity is defined as ‘‘. . . any activity (including the cultivation of any particular crop or other vegetation) . . . [that] . . . is likely to reduce the availability of water in a watercourse to the Reserve, to meet international obligations, or to other water users significantly’’ (NWA Section 36(2)). As a declared water-user, forestry is subject to a wateruse entitlement, similar to all the other uses. Licences are
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Fig. 2. Spatial variation of E in the upper Thukela catchment and surrounding areas for 2nd March 2001 computed using the SEBAL algorithm Kongo and Jewitt (2006).
based on estimates of water use per management area, typically a quaternary catchment,1 and licenses are granted or declined according to an analysis of all of that catchments water user needs and whether the proposed use is deemed ‘‘beneficial’’. The focus on low flows as it is in the dry season that evergreen vegetation is likely to have the greatest impact (Table 1). The intention of introducing the SFRA concept was to subject all users of water, in whichever form it is used, to the same regulation. Thus, whether water use is in the form of blue water or Green Water Flows, the same regulations do, in principle, apply (Jewitt, 2002b; Calder, 2005). Difficulties with the implementation of the SFRA policy range from administrative to political to hydrological, but are not regarded as insurmountable and much progress has been made in implementing it. The factors considered critical in the considering whether a land use should be declared an SFRA or not are the extent of stream flow reduction, its duration, and its impact on any relevant water resource and on other water users. Consequently, although much attention has
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The SA Department of Water Affairs and Forestry subdivides the countries catchments to the fourth level of breakdown i.e. the quaternary catchment, for water resources management purposes.
been paid to providing estimates and direct measurement of Green Water Flows from various land uses, the consideration of other users and links to the impacts on blue water resources has been a focus of attention. Currently, the ACRU Agrohydrological Model is the preferred tool for estimating runoff reduction due to commercial forestry (Jewitt and Schulze, 1999; Gush et al., 2002). 3.2. Runoff harvesting and related innovations Water focused innovations in agriculture have been promoted as means of storing and conserving water resources in different mediums for agricultural, domestic and livestock use. In the case of crop production, these are usually intended to conserve and preserve sufficient soil moisture in the root zone for crop uptake during transpiration. This is achieved by encouraging infiltration and reducing soil evaporation (e.g., conservation tillage) and by storing surface runoff for supplemental irrigation during dry spells. Typically, the rationale for adapting and adopting these water use system innovations is that, though seasonal amounts of rainfall (monthly and annual) may be adequate for crop production, its temporal distribution within the growing season is poor and soil moisture deficits which will
Table 1 A conceptual model of the hydrological response of land use on different aspects of low flow relative to a grassland baseline (IAP – Invasive Aliens Plants) Impact on wet season rechargea
Impact on onset of wet season flowsb
Impact on onset of dry season (‘‘normal’’ years) flowsc
Impact on onset of dry season (‘‘dry’’ years) flowsd
Direct utilization of groundwatere
Long term impact on groundwater reservesf
Eucalyptus
Yes – likely to be high
Yes – likely to be high
Yes – moderate impact
Yes – likely to be high
Possible in some situations
Pine Wattle Sugar cane
Yes Yes Possible – low impact Possible – low impact Possible – low impact Low Low
Yes – likely to be high Yes – likely to be high Yes – higher soil moisture deficits than natural. Moderate impact likely Minimal impact – especially where conservation tillage is practised Minimal impact – especially where conservation tillage is practised Yes – likely to be high Early runoff reduced downstream. Could be high depending on extent Yes – likely to be high
Low impact Low impact Low impact
Yes – likely to be high Yes – likely to be high Yes – higher soil moisture deficits than natural. Moderate impact likely Moderate impact – especially where conservation tillage is practised Moderate impact – especially where conservation tillage is practised Yes – likely to be high Could be high depending on extent
Rare Rare Rare – localized perched water tables only No
High due to combination of impact of recharge and direct abstraction Moderate Moderate Low to moderate
Yes – moderate impact
Yes – likely to be high
Yes – higher soil moisture deficits than natural. Moderate impact likely. Possibly higher where RZ is impacted Low to moderate depending tree characteristics (leaf and rooting patterns) and stage of growth Yes – likely to be high
Maize Dryland wheat Farm dams Runoff harvesting IAP – tall vegetation
Yes – likely to be high
Low impact Low impact Low impact Low impact
IAP – short vegetation
Possible – low impact
Yes – higher soil moisture deficits than natural. Moderate impact likely. Possibly higher where RZ is impacted
Low impact
Natural forest
Low impact
Low
Irrigation*
Often positive due to return flows
Low – slightly higher soil moisture deficits in cases of non-deciduous trees Yes – likely to be high
Low impact – could be positive
Low
No
Low
No No
Low Low
Possible in some situations. Impact could be very high in situation where RZ is impacted Rare – localized perched water tables only
High due to combination of impact of recharge and direct abstraction Moderate
On in riparian area, or other areas where groundwater table is shallow No
Low
Low
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Land use
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Note that responses from irrigated land differ fundamentally from dryland activities. The responses here highlight the possible impact of the field, not those of the abstraction and storage systems. Extent to which wetting fronts move beyond root zone in wet season. b Extent to which elevated flows following onset of summer rains will be delayed and reduced. c Extent to which elevated flows from summer will persist through autumn (summer rainfall areas) and spring (winter rainfall areas) following a wet season in which the rainfall could be considered ‘‘normal’’ or close to the long term mean. d Extent to which elevated flows from summer will persist through autumn (summer rainfall areas) and spring (winter rainfall areas) following a wet season in which the rainfall could be considered drier than normal. e Extent to which land use may be able to utilize groundwater directly. f Extent to which land use may ‘‘mine’’ water and hence have severe and delayed impacts on low flow generation. a
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limit yields occur during crucial stages of crop. Careful management of moisture in the root zone thus provides opportunities to mitigate periods of soil water stress in the crop growing cycle. Although, such approaches are not new, the spatial and temporal impacts of such water use innovations are rarely assessed. Many proponents of this approach, it is considered that such innovations contribute to an increase in the recharge of water to the root zone and finally to groundwater. However, the likelihood is that this water, now stored in the soil will not move beyond the root zone, will not contribute to groundwater recharge and will not be available to downstream users. Ro¨ckstrom et al. (2004) noted that if such water system innovations are successfully implemented, there will be many interacting implications for biophysical, economic, and ecological systems, and highlighted the role of specific research programmes in addressing such concerns. Two such programmes are the SSI Programme in the Thukela and Pangani Catchments (Ro¨ckstrom et al., 2004) and the WATERNET led Challenge Programme in the Limpopo Basin (Love et al., 2004). Both programmes include studies which consider the role of different biophysical disciplines within a catchment (e.g., upstream–downstream hydrological impacts and general environment) and linkages between the agro-ecological system and society within the focal catchments. It is considered critical to understand these interactions and implications at different spatial and temporal scales before promotion and adoption of such water use innovations in a catchment and river basin at large.
4. The consideration of green water flows in water resources management and planning 4.1. Difficulties in integrating blue and green water flows The storage of both saturated and unsaturated water in the soil profile and the partitioning of this water to evaporation from the soil, transpiration, groundwater recharge and to different parts of the downstream flow regime is the least understood aspect of the hydrological cycle. Despite very strong perceptions, the extent to which groundwater situated in aquifers or water derived from unsaturated flow in soils contribute to baseflows and even stormflows is not well known. Recent isotope studies indicate that high levels of water from unsaturated soil water are found in baseflows – contrary to the perceptions of many (Lorentz et al., 2003). Inevitably, strong perceptions are formed depending upon the scale at which the associated processes are observed. Very importantly, and an issue which is not given the attention it deserves is that of temporal scale, i.e. impacts may accumulate over time. In considerations of human and environmental flow requirements, the extent of impact, its duration, and impact on other water users are critical considerations in assessing the impact of
changes in Green Water Flows associated with changes in land use. Long term averages applicable to large spatial scales are typically produced to highlight the importance of Green Water Flows (e.g., Rockstro¨m et al., 1999; Jewitt et al., 2004; Calder et al., 2005; Schulze et al., 2005). Furthermore, many argue that over ‘‘long periods’’, the change in catchment storage tends to zero and that increased Green Water Flows can be directly linked to impacts on blue water resources where changes in soil water are assumed to be zero. Consequently, in such analyses, it appears that there is a direct link between the land use change under consideration and changes in both blue and green water responses. However, at smaller temporal scales, these links are not direct and when considering environmental and human water requirements, it is imperative that water resources management and planning must consider appropriate spatial and temporal scales. The role of the storage of water in the catchment soil profile is critical in these considerations. Furthermore, many initiatives to protect groundwater are focused on limiting abstraction. However, in arid and semi-arid regions, particularly where deep soils exist, the impact to which a change in land use limits recharge to the aquifer may be more important. In the case of large scale land use change, such as the conversion of seasonally growing grasslands to rapidly growing short rotation commercial forestry, recharge to groundwater will only occur in abnormally wet periods. In these cases low flows will be profoundly and permanently impacted upon, as the soil profile will be progressively dried out over time and these impacts will only become obvious over relatively long temporal scales (Dye, 1996). In the Free State Province of SA where mean annual precipitation is in the order of 500 mm/a, dryland agricultural practices have evolved such that crops are not planted until such time as approximately 250 mm of water are stored in the soil. Once this has occurred crops (typically maize or wheat) are planted. In situ runoff harvesting has also been widely adopted in these areas (Hensley et al., 2000). In these cases, small runon plots are established between crop rows and water is captured and stored in the soil just upslope of the row of plants. In the Bijapur District of Southern India, (Calder et al., 2005) report that downstream users are impacted on when blue water flows are reduced by a proliferation of RWH systems which aim to optimize the potential for Green Water Flows. Thus, impacts on the generation of low flows will be largely dependent on the extent to which the land use will utilize water in the soil horizon which could contribute directly to the generation of low flows and indirectly by limiting groundwater recharge. Statements which highlight soil moisture as an ‘‘underutilized resource’’ (Falkenmark and Ro¨ckstrom, 2005) should, therefore, be treated with caution. The extent to which these impacts may occur is dependent upon a number of issues, as highlighted in Table 1 which provides a conceptual overview of possible impacts
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of changing land cover on a catchments response, with a focus on runoff downstream, relative to a baseline condition assumed to represent a natural grass. Although much of the information in this table is conceptual, it does provide a ‘‘strawdog’’ or point of discussion, where a wider body of experience and research may be able to expand on the ideas presented. Thus, the simulation of how vegetation affects the primary process of evaporation from a catchment rather than the secondary consequences on stream flow is attractive as the simulation process is simpler and the tools available to both measure and predict (model) green water flows can be applied with greater confidence than those for streamflow. There are a number of reasons for this, the most significant being that by focusing on estimates of E, the uncertainties introduced by considering a second dimension (i.e. lateral flow) are avoided. Thus, where accuracy and confidence in the estimates of water use of one crop relative for long term planning is required, the ‘‘green water’’ approach does offer some advantages. However, as highlighted above, long term means of estimated Green Water Flows applied to large spatial scales fall short as management tools. People and ecosystems react at small spatial scales and to short term events such as intra seasonal dry periods. This is particularly true in semi-arid regions where many water resources management issues, such as planning for environmental flows and human water requirements require an understanding of the impact of land use on the flow regime in its entirety. 4.2. Appropriate water resources management and planning tools A dilemma in highlighting the role of Green Water Flows in IWRM exists around the extent to which the information can be simplified, and yet retain a management as well as an illustrative role. As highlighted above, many studies have provided estimates of long term average green and blue water flows in response to land use (Jewitt et al., 2004; Calder et al., 2005; Falkenmark and Ro¨ckstrom, 2004) and climate change (Schulze et al., 2005) at broad spatial and long temporal scales. Whilst these can be used to illustrate the potential impacts of land use change and focus attention on the potential for the management of land use i.e. Green water management tools, these are rarely useable as tools to address the detail required to ensure that daily downstream flow requirements are met. Where such information is needed, these are integrated within hydrological models with varying degrees of complexity and success. As a first step towards the consideration of downstream impacts of RWH in India, Calder et al. (2005) have produced a ‘‘Quadrant’’ approach where, quadrants represent the potential for land use change to cause Green Water Flows to exceed incoming precipitation and where prescribed minimum blue water flows may not be met (Fig. 3). This approach does provide a useful means of
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Fig. 3. ‘‘Quadrant’’ approach where catchment conditions which can be used to identify green and blue water management options and whether benefits would be derived from further soil water conservation measures (after Calder et al., 2005).
highlighting the link between green and blue water flows, especially in a context where consideration of environmental flows and other downstream users is extremely limited. However, this is based on an assumption that there is a direct causal relationship between Green Water Flows and runoff; an assumption that is only valid at large temporal scales and as such is inadequate as a tool to address the day to day concerns of water resources managers and planners. Flow Duration Curves (FDCs) have long been used in assessing assurance of supply and risk associated with the reticulation and supply of water, particularly where large water storage systems are being considered. FDC’s can be derived from both observed and simulated time series and have been shown to be one of the most informative methods of displaying the entire flow record. However, their use when considering run of river and smaller catchment scale situations has been limited. In South Africa, FDCs have been used to integrate the methodologies to assess, estimate and illustrate environmental flows (Hughes and Hannart, 2003) and have been investigated in assessments of commercial afforestation on streamflow (Scott et al., 2000). Recent publications by Brown et al. (2005) and Lane et al. (2005) have highlighted the usefulness of using FDC when assessing the impact of land use change on the flow regime. The use of FDCs also provides a means of assessing risk. In SA SFRA assessments are historically precautionary in favour of the low flow periods (Gush et al., 2002). However, integration of these periods with estimates of environmental flows using the methods adopted by Hughes and Hannart (2003) provides a far more sophisticated means of assessing a potential impact relative to a required assurance of supply for the downstream users or other users in the spatial unit under consideration. In this approach time series of output from hydrological models can be subjected
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to analysis of the percentage of time that the recommended flows to meet environmental or human needs are equaled or exceeded. In South Africa such human and environmental water requirements are expressed in terms of the Reserve – or the amount of blue water required downstream and are derived from numerous studies at different levels of detail. It is accepted that the Reserve amount is not static. It is not a minimum flow requirement, but rather a volume which varies according time of year and which can be expressed as a relative portion of the flow regime rather than in absolute terms. The combination of these tools provides for a framework for water resources planners in the country by which the impact of Green Water Flows by commercial afforestation in South Africa can be assessed (Hughes, 2006). Within this framework, time series from the ACRU Model, capable of detailed representation of green and
blue water flows, representing the likely impact of commercial afforestation can be plotted on the same graph as FDCs representing a reference or naturalized flow, present day flows and the Reserve as illustrated by Figs. 4 and 5. The y-axis of the FDC represents the percentage of natural flow remaining, and plots of this curve will decline with increasing Green Water Flows until the Reserve threshold is reached. Fig. 5 is typical of these analyses in that the lines first intersect at the low flow periods of the flow regime. The use of the SPATSIM framework (Hughes and Palmer, 2005) for this purpose enables direct links between the Quaternary Scale Reserve Determination Model representing blue water flows and SFRAs (green water flows) to be established. Such analyses can provide an expression of the assurance with which such target flows can achieve and thus provide information which is extremely important in water
Fig. 4. The likely impact of different scenarios can be assessed for different months. The model can then be applied to assess the likely impact of increasing the area of the intended tree type (or other similar analyses). Areas where the future flow and Resrve FDC intersect may be problematic.
Fig. 5. The impact can be assessed for different levels of assurance of supply. Different levels of assurance of supply for months of interest can be selected and assessed.
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resources planning and management (Hughes, 2006). This approach also effectively provides an equivalent to the assurance criteria that are typically used to quantify the reliability of a component of a water supply project. Thus in integrating Green Water Flows with blue water management approaches, time series derived from SEBAL or similar remotely sensed or model derived estimates of Green Water Flows can be used as input or components of integrated hydrological modelling systems which in turn can provide FDC’s for assessment of land use impacts on blue water users, including the Reserve, for the required assurance of supply.
5. Discussion/conclusions Green Water Flows should be more explicitly considered in water resources planning and management. However, as highlighted by Falkenmark and Ro¨ckstrom (2005) when interacting with water resources policy makers and planners, we are often restricted to a ‘‘sanctioned discourse’’ which is focused on how existing blue water resources can best be utilized or reallocated and in which there is very little consideration of the broader hydrological cycle. Such a discourse is in danger of perpetuating poor planning, particularly if reallocation of water needs are not more closely linked to land allocation. Although this may seem obvious, these are often separated in policy. A focus on green water, although potentially acceptable for illustrative purposes cannot be used to manage impacts on blue water unless targets or thresholds are set with unrealistically high safety margins. This is required to minimize risks associated with uncertainties regarding the links between green and blue water flows and the generation of flows, particularly low flows in rivers. The challenge to scientists is to provide a means of assessing Green Water Flows in a context that is useable to policy makers. Thus, the use of innovative approaches such the estimations of Green Water Flows from remote sensing which provides a means of understanding spatial and temporal distribution of Green Water Flows and ultimately a means of testing the E component of hydrological models should be encouraged and how this information is presented to policy makers should be refined. Linkages between this information and blue water flows needs to be made, and the use of flow duration curves is a useful means of linking with ‘‘traditional’’ water resources management and planning approaches. Acknowledgements This paper draws from ongoing research which is supported by SIDA, IWMI and IHE through the SSI Programme and the Water Research Commission of South Africa through Project K5/1428.
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terrestrial ecosystem services. Conservation Ecology 3 (2), 5 [online] http://www.consecol.org/vol3/iss2/art5. Ro¨ckstrom, J., Folke, C., Gordon, L., Hatibu, N., Jewitt, G., Penning de Vries, F., Rwehumbiza, F., Sally, H., Savenije, H., Schulze, R., 2004. A watershed approach to upgrade rainfed agriculture in water scarce regions through Water System Innovations: an integrated research initiative on water for food and rural livelihoods in balance with ecosystem functions. Physics and Chemistry of the Earth 29 (15–18), 1109–1118. Savenije, H.H.G., 2004. The importance of interception and why we should delete the term evapotranspiration from our vocabulary. Hydrological Processes 18 (8), 1507–1511. Schulze, R.E., Horan, M.J.C., Gray, R., 2005. Sensitivity studies of hydrological responses in the Thukela catchment to spatial and temporal representations when using a baseline and a projected future climate scenario. In: Schulze, R.E. (Ed.), Climate Change and Water Resources in Southern Africa: Studies on Scenarios, Impacts, Vulnerabilities and Adaptation. Water Research Commission, Pretoria, RSA, WRC Report 1430/1/05. pp. 211–232 (Chapter 11). Scott, D.F., Prinsloo, F.W., Moses, G., Mehlomakulu, M., Simmers, A.D.A., 2000. A re-analysis of the South African catchment afforestation experimental data. Water Research Commission, Pretoria, Report 810/1/00, p. 138. Wiles, L.W., Blight, J.B., Everson, C.E., Jewitt, G.P.W., 2005. Total evaporation estimation of sugarcane using the scintillation technique. In: Proceedings of the 11th Conference of the South African Council of the Institute of Applied Hydrological Scientists, Midrand, September 2005.
Integrating blue and green water flows for water resources management and planning Graham Jewitt
*
School of Bioresources Engineering and Environmental Hydrology, University of KwaZulu-Natal, South Africa
Abstract The ‘‘Green Water’’ approach, where flows of water vapour in the form of transpiration, interception and evaporation from the soil and vegetation is considered green water and runoff and groundwater recharge is considered blue water, has been an extremely useful illustrative concept in many situations where the role of land use in water resources management needs to be highlighted. The approach has been the subject of much interest in recent years, particularly in semi-arid and arid regions where Green Water Flows dominate the hydrological cycle. However, it is clear that there are limits to the concept in informing water resources management and planning. In this paper, these limits are explored through case studies of commercial afforestation and runoff harvesting in the SADC region. Issues highlighted include the degree of simplification of the hydrological cycle in many green water focused studies, appropriate spatial and temporal scales for the consideration of low flows and the uncertainty regarding the storage of water in the soil profile and the generation of flows from saturated and unsaturated soil water. It is concluded that rather than focusing on green or blue water flows, it is the hydrological linkages between these and their representation in water resources management and planning that needs most attention. 2006 Elsevier Ltd. All rights reserved. Keywords: Green Water Flows; Integrated water resources management; Afforestation; Rainwater harvesting
1. Introduction The ‘‘Green Water’’ approach has gained prominence since it was initially introduced by Falkenmark in 1995 (Falkenmark, 1995). The promotion of Green Water Flows has been extremely effective in highlighting the role of total evaporation (E) from the landscape in the hydrological cycle and the potential to utilize the transpiration portion of these flows for food and fibre production. However, although the concept has been extremely useful in this regard, the consideration of Green Water Flows in formal water resources management and planning is proving to be extremely difficult. Green Water Flows have been the subject of much interest in recent years, particularly in semi-arid and arid
*
Tel.: +27 33 2605678; fax: +27 33 2605818. E-mail address: [email protected]
1474-7065/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pce.2006.08.033
regions where they dominate the hydrological cycle. In these areas, small changes in Green Water Flows can result in major impacts on downstream blue water flows. As highlighted, by Falkenmark and Ro¨ckstrom (2005), this provides a major dilemma in that meeting food requirements for future populations will require additional utilization of land and therefore, water resources and that conflict with other users, in particular users who are reliant on run of river abstractions, is inevitable. These users are typically the most vulnerable, e.g., rural communities and aquatic ecosystems. Historically and inevitably water resources planning management is focused on blue water. If Green Water Flows are to be considered explicitly in future water resources management, it is imperative that the land segment i.e. the catchment is more explicitly considered in water resources management. This cuts to the heart of some of the debate of the mid-1990s which considered Integrated Catchment Management vs. Integrated Water Resources Management (IWRM). In essence, much
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of the debate was about the extent to which management of the catchment i.e. the landscape could be included in approaches which claimed to provide for Integrated Water Resources Management. In South Africa, it is recognized that the ultimate goal is Integrated Catchment Management in which all water users, including non-irrigated land use are considered, but it was accepted by DWAF and WRC (1998) that IWRM in South Africa’s immediate future would ‘‘comprise mutual and sensitive dependence of water, land-use and aquatic ecology management, but with incomplete integration of natural resource management’’. Consequently true Integrated Catchment Management remains a very distant goal, although IWRM continues with a catchments or watersheds as a fundamental management unit (Jewitt, 2002a). Increasingly, scientists are realizing that the consideration of Green Water Flows in water resources management will require a higher level of integration which explicitly considers land issues together with water issues, sometimes termed ‘‘Integrated Land and Water Management’’ (Berry et al., in press) or ‘‘Integrated Land and Water Resources Management’’ (ILWRM) (Falkenmark and Ro¨ckstrom, 2004). It has been suggested that ILWRM may require policies that are more explicitly directed at the management of the land resource as it affects evaporation and water resources – i.e. Green Water Policy Instruments (Calder, 2005). But what form will such instruments take, and do any exist? Whilst it can be argued that Green Water Flows have not been adequately considered, water resources management and planning must consider the potential impacts on blue water flows. The hydrological difficulties pertaining to the consideration of Green Water Flows in policy arise from two issues: • The difficulties in measurement or estimation of wateruse of a land based activity. • Understanding and predicting the links between Green and Blue water flows and the spatial and temporal responses of catchments. Whilst from a water resources management and planning perspective, the primary issues arise from: • Difficulties in managing the landscape within water resources management legislation which is inevitably focused on blue water. Thus, these three issues are highlighted in this paper. 2. Quantifying green water flows Most often Green Water Flows are considered to represent Total Evaporation, (often called ‘‘evapotranspiration’’) and are considered to comprise both evaporation and transpiration components. The evaporation component is made up of evaporation of intercepted water evap-
oration from plant surfaces as well as free water surfaces, and evaporation from the soil. This component is considered non-productive and in order to distinguish it from transpiration, it is sometimes been referred to as ‘‘white water’’ (Savenije, 2004) highlighting the concern that, hydrologically, it is problematic to lump these two components. Although they interlinked, the processes governing their generation are different and despite the attractiveness and simplicity of the term ‘‘Green Water Flows’’, failure to highlight the different components making up E may compromise hydrological modelling efforts where, it is argued in this paper, good conceptual understanding is prerequisite for integrating blue and green water flows for water resources management. However, in order not to weaken the fundamental message of the green–blue approach the original and most commonly applied terminology i.e., where these are considered productive and non-productive Green Water Flows (Falkenmark and Ro¨ckstrom, 2004), is applied here with the clear recognition that these need to be considered separately in hydrological process studies. Indeed, it is the movement of water through agricultural crops and timber by transpiration (i.e. productive water use) that must be optimized if future food and fibre requirements are to be met, hence the evolution of the phrase ‘‘more crop per drop’’. There are several challenges associated with estimating or measuring Green Water Flows. In the first instance, difficulties in determining the spatial and temporal variation of E over large areas are compounded by the many factors that influence its occurrence and prevalence. These include local micro-climatic effects as well as different vegetation and soil types. The conventional approaches for quantifying E have been based on localized point measurements and do not allow for its estimation over large geographical areas. These approaches include direct measurements (evaporation pans, lysimeters, etc.), climatic stations (eddy covariance, Bowen ratio, etc.), heat pulse systems (transpiration only) and hydrological models (water balance and biomass production). Secondly, there are very few methods which allow for estimates of transpiration and evaporation separately. The Heat Pulse method is very useful in woody vegetation (e.g., Dye and Olbrich, 1993) but has proven problematic in herbaceous plants. Many Soil-VegetationAtmosphere models exist and usually, the total evaporation process is represented by some derivative of the Penman equation to separate evaporation into its components of interception, soil evaporation and transpiration. Notwithstanding this recognition, it is argued that effective and accurate estimation of Green Water Flows at large spatial scales as opposed to the more ‘‘traditional’’ point scale measurements will be highly beneficial to scientists as well as water resources managers and planners. They will provide both direct estimates of E as well providing data which can be used for the improvement of existing, and development of new models. In this regard, two of
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the most of the promising methods are the application of the scintillation method and remote sensing for estimates of E using the surface energy balance. 2.1. Estimation of Total Evaporation by scintillation Scintillation is a general term used to describe the changes in brightness of an object when viewed through the atmosphere. As an electromagnetic wave moves through the atmosphere it will be distorted by refraction, absorption and diffraction resulting in intensity fluctuations caused by heat, moisture and pressure variations. The Large Aperture Scintillometer is an instrument used to measure such fluctuations and operates at scales from 500 to 5000 m. The signal is transmitted by a light source and received by a receiver across a transect selected by the operator (Fig. 1). Additional data on temperature, pressure and humidity are necessary to compute sensible heat flux and ultimately estimate evaporation. Details of the method are provided by Kongo and Jewitt (2006). An important feature of the scintillation technique is that, although the measurement is along a path of a light beam, this actually provides an estimate of evaporation over an area because of the wide fetch (Meijninger and Bruin, 2000). In South Africa, three such instruments exist and are increasingly being used to estimate Green Water Flows from a variety of land uses including Sugar Cane (Wiles et al., 2005), Indigenous Forest (Dye, P. Council for Scientific and Industrial Research, Personal Communication, 2005) and rehabilitated mine dumps (Jarmain C. Council for Scientific and Industrial Research, Personal Communication, 2005). 2.2. Remote sensing of evaporation Remote sensing data provided by satellites provides a means of obtaining consistent and frequent observations
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of spectral reflectance and emmitance of radiation from the land surface at scales ranging from 30 m and larger. Various methods have been derived in order to estimate E from these spectral radiances, probably the most well known of which is the Surface Energy Balance Algorith (SEBAL) (Bastiaanssen et al., 1998a). SEBAL is an energy partitioning algorithm comprised of 25 computational submodels that calculates E and other energy exchanges at the earth’s surface (Bastiaanssen et al., 1998a,b). The algorithm computes most essential hydro-meteorological parameters and does require limited ground based meteorological data. It has been applied recently in the Thukela Catchment (Kongo and Jewitt, 2006) and the Limpopo Basin (Ahmad et al., 2005). Based on available measured E, initial results obtained seem reasonable, but also of importance is the spatial variability in Green Water Flows which can be derived from the method as illustrated in Fig. 2. One of the intermediate outputs of the SEBAL algorithm is the sensible heat flux for each pixel of the satellite image used. This provides an opportunity to validate SEBAL output using measured sensible heat flux data obtained from a Large Aperture Scintillometer as described above. 3. Impacts of land use change and green water flows The consideration of the land use change in water resources management and planning is problematic. However, in South Africa, because of its high levels of green water use and resulting impact on downstream blue water resources, the establishment of commercial afforestation has been controlled for over 30 years. Thus, the potential for impacts on the water resource resulting from a change in land use are well known and consequently the enthusiasm to embrace Rainwater Harvesting (RWH) approaches is tempered with some concern regarding whether there are likely to be downstream impacts on blue water flows. 3.1. Streamflow reduction activities (SFRA): a step towards green water policy?
Fig. 1. A Large Aperture Scintillometer in operation above a sugar cane transect.
In South Africa, forestry has historically been recognized as an important water user and concerns for the protection of water resources have led to the control of commercial afforestation and forestry practices since 1972. Under the National Water Act (NWA) (Act no. 36 of 1998) forestry is considered a ‘‘stream flow reduction activity’’ (SFRA) and as such must be licensed as a water user. A stream flow reduction activity is defined as ‘‘. . . any activity (including the cultivation of any particular crop or other vegetation) . . . [that] . . . is likely to reduce the availability of water in a watercourse to the Reserve, to meet international obligations, or to other water users significantly’’ (NWA Section 36(2)). As a declared water-user, forestry is subject to a wateruse entitlement, similar to all the other uses. Licences are
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Fig. 2. Spatial variation of E in the upper Thukela catchment and surrounding areas for 2nd March 2001 computed using the SEBAL algorithm Kongo and Jewitt (2006).
based on estimates of water use per management area, typically a quaternary catchment,1 and licenses are granted or declined according to an analysis of all of that catchments water user needs and whether the proposed use is deemed ‘‘beneficial’’. The focus on low flows as it is in the dry season that evergreen vegetation is likely to have the greatest impact (Table 1). The intention of introducing the SFRA concept was to subject all users of water, in whichever form it is used, to the same regulation. Thus, whether water use is in the form of blue water or Green Water Flows, the same regulations do, in principle, apply (Jewitt, 2002b; Calder, 2005). Difficulties with the implementation of the SFRA policy range from administrative to political to hydrological, but are not regarded as insurmountable and much progress has been made in implementing it. The factors considered critical in the considering whether a land use should be declared an SFRA or not are the extent of stream flow reduction, its duration, and its impact on any relevant water resource and on other water users. Consequently, although much attention has
1
The SA Department of Water Affairs and Forestry subdivides the countries catchments to the fourth level of breakdown i.e. the quaternary catchment, for water resources management purposes.
been paid to providing estimates and direct measurement of Green Water Flows from various land uses, the consideration of other users and links to the impacts on blue water resources has been a focus of attention. Currently, the ACRU Agrohydrological Model is the preferred tool for estimating runoff reduction due to commercial forestry (Jewitt and Schulze, 1999; Gush et al., 2002). 3.2. Runoff harvesting and related innovations Water focused innovations in agriculture have been promoted as means of storing and conserving water resources in different mediums for agricultural, domestic and livestock use. In the case of crop production, these are usually intended to conserve and preserve sufficient soil moisture in the root zone for crop uptake during transpiration. This is achieved by encouraging infiltration and reducing soil evaporation (e.g., conservation tillage) and by storing surface runoff for supplemental irrigation during dry spells. Typically, the rationale for adapting and adopting these water use system innovations is that, though seasonal amounts of rainfall (monthly and annual) may be adequate for crop production, its temporal distribution within the growing season is poor and soil moisture deficits which will
Table 1 A conceptual model of the hydrological response of land use on different aspects of low flow relative to a grassland baseline (IAP – Invasive Aliens Plants) Impact on wet season rechargea
Impact on onset of wet season flowsb
Impact on onset of dry season (‘‘normal’’ years) flowsc
Impact on onset of dry season (‘‘dry’’ years) flowsd
Direct utilization of groundwatere
Long term impact on groundwater reservesf
Eucalyptus
Yes – likely to be high
Yes – likely to be high
Yes – moderate impact
Yes – likely to be high
Possible in some situations
Pine Wattle Sugar cane
Yes Yes Possible – low impact Possible – low impact Possible – low impact Low Low
Yes – likely to be high Yes – likely to be high Yes – higher soil moisture deficits than natural. Moderate impact likely Minimal impact – especially where conservation tillage is practised Minimal impact – especially where conservation tillage is practised Yes – likely to be high Early runoff reduced downstream. Could be high depending on extent Yes – likely to be high
Low impact Low impact Low impact
Yes – likely to be high Yes – likely to be high Yes – higher soil moisture deficits than natural. Moderate impact likely Moderate impact – especially where conservation tillage is practised Moderate impact – especially where conservation tillage is practised Yes – likely to be high Could be high depending on extent
Rare Rare Rare – localized perched water tables only No
High due to combination of impact of recharge and direct abstraction Moderate Moderate Low to moderate
Yes – moderate impact
Yes – likely to be high
Yes – higher soil moisture deficits than natural. Moderate impact likely. Possibly higher where RZ is impacted Low to moderate depending tree characteristics (leaf and rooting patterns) and stage of growth Yes – likely to be high
Maize Dryland wheat Farm dams Runoff harvesting IAP – tall vegetation
Yes – likely to be high
Low impact Low impact Low impact Low impact
IAP – short vegetation
Possible – low impact
Yes – higher soil moisture deficits than natural. Moderate impact likely. Possibly higher where RZ is impacted
Low impact
Natural forest
Low impact
Low
Irrigation*
Often positive due to return flows
Low – slightly higher soil moisture deficits in cases of non-deciduous trees Yes – likely to be high
Low impact – could be positive
Low
No
Low
No No
Low Low
Possible in some situations. Impact could be very high in situation where RZ is impacted Rare – localized perched water tables only
High due to combination of impact of recharge and direct abstraction Moderate
On in riparian area, or other areas where groundwater table is shallow No
Low
Low
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Land use
*
Note that responses from irrigated land differ fundamentally from dryland activities. The responses here highlight the possible impact of the field, not those of the abstraction and storage systems. Extent to which wetting fronts move beyond root zone in wet season. b Extent to which elevated flows following onset of summer rains will be delayed and reduced. c Extent to which elevated flows from summer will persist through autumn (summer rainfall areas) and spring (winter rainfall areas) following a wet season in which the rainfall could be considered ‘‘normal’’ or close to the long term mean. d Extent to which elevated flows from summer will persist through autumn (summer rainfall areas) and spring (winter rainfall areas) following a wet season in which the rainfall could be considered drier than normal. e Extent to which land use may be able to utilize groundwater directly. f Extent to which land use may ‘‘mine’’ water and hence have severe and delayed impacts on low flow generation. a
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limit yields occur during crucial stages of crop. Careful management of moisture in the root zone thus provides opportunities to mitigate periods of soil water stress in the crop growing cycle. Although, such approaches are not new, the spatial and temporal impacts of such water use innovations are rarely assessed. Many proponents of this approach, it is considered that such innovations contribute to an increase in the recharge of water to the root zone and finally to groundwater. However, the likelihood is that this water, now stored in the soil will not move beyond the root zone, will not contribute to groundwater recharge and will not be available to downstream users. Ro¨ckstrom et al. (2004) noted that if such water system innovations are successfully implemented, there will be many interacting implications for biophysical, economic, and ecological systems, and highlighted the role of specific research programmes in addressing such concerns. Two such programmes are the SSI Programme in the Thukela and Pangani Catchments (Ro¨ckstrom et al., 2004) and the WATERNET led Challenge Programme in the Limpopo Basin (Love et al., 2004). Both programmes include studies which consider the role of different biophysical disciplines within a catchment (e.g., upstream–downstream hydrological impacts and general environment) and linkages between the agro-ecological system and society within the focal catchments. It is considered critical to understand these interactions and implications at different spatial and temporal scales before promotion and adoption of such water use innovations in a catchment and river basin at large.
4. The consideration of green water flows in water resources management and planning 4.1. Difficulties in integrating blue and green water flows The storage of both saturated and unsaturated water in the soil profile and the partitioning of this water to evaporation from the soil, transpiration, groundwater recharge and to different parts of the downstream flow regime is the least understood aspect of the hydrological cycle. Despite very strong perceptions, the extent to which groundwater situated in aquifers or water derived from unsaturated flow in soils contribute to baseflows and even stormflows is not well known. Recent isotope studies indicate that high levels of water from unsaturated soil water are found in baseflows – contrary to the perceptions of many (Lorentz et al., 2003). Inevitably, strong perceptions are formed depending upon the scale at which the associated processes are observed. Very importantly, and an issue which is not given the attention it deserves is that of temporal scale, i.e. impacts may accumulate over time. In considerations of human and environmental flow requirements, the extent of impact, its duration, and impact on other water users are critical considerations in assessing the impact of
changes in Green Water Flows associated with changes in land use. Long term averages applicable to large spatial scales are typically produced to highlight the importance of Green Water Flows (e.g., Rockstro¨m et al., 1999; Jewitt et al., 2004; Calder et al., 2005; Schulze et al., 2005). Furthermore, many argue that over ‘‘long periods’’, the change in catchment storage tends to zero and that increased Green Water Flows can be directly linked to impacts on blue water resources where changes in soil water are assumed to be zero. Consequently, in such analyses, it appears that there is a direct link between the land use change under consideration and changes in both blue and green water responses. However, at smaller temporal scales, these links are not direct and when considering environmental and human water requirements, it is imperative that water resources management and planning must consider appropriate spatial and temporal scales. The role of the storage of water in the catchment soil profile is critical in these considerations. Furthermore, many initiatives to protect groundwater are focused on limiting abstraction. However, in arid and semi-arid regions, particularly where deep soils exist, the impact to which a change in land use limits recharge to the aquifer may be more important. In the case of large scale land use change, such as the conversion of seasonally growing grasslands to rapidly growing short rotation commercial forestry, recharge to groundwater will only occur in abnormally wet periods. In these cases low flows will be profoundly and permanently impacted upon, as the soil profile will be progressively dried out over time and these impacts will only become obvious over relatively long temporal scales (Dye, 1996). In the Free State Province of SA where mean annual precipitation is in the order of 500 mm/a, dryland agricultural practices have evolved such that crops are not planted until such time as approximately 250 mm of water are stored in the soil. Once this has occurred crops (typically maize or wheat) are planted. In situ runoff harvesting has also been widely adopted in these areas (Hensley et al., 2000). In these cases, small runon plots are established between crop rows and water is captured and stored in the soil just upslope of the row of plants. In the Bijapur District of Southern India, (Calder et al., 2005) report that downstream users are impacted on when blue water flows are reduced by a proliferation of RWH systems which aim to optimize the potential for Green Water Flows. Thus, impacts on the generation of low flows will be largely dependent on the extent to which the land use will utilize water in the soil horizon which could contribute directly to the generation of low flows and indirectly by limiting groundwater recharge. Statements which highlight soil moisture as an ‘‘underutilized resource’’ (Falkenmark and Ro¨ckstrom, 2005) should, therefore, be treated with caution. The extent to which these impacts may occur is dependent upon a number of issues, as highlighted in Table 1 which provides a conceptual overview of possible impacts
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of changing land cover on a catchments response, with a focus on runoff downstream, relative to a baseline condition assumed to represent a natural grass. Although much of the information in this table is conceptual, it does provide a ‘‘strawdog’’ or point of discussion, where a wider body of experience and research may be able to expand on the ideas presented. Thus, the simulation of how vegetation affects the primary process of evaporation from a catchment rather than the secondary consequences on stream flow is attractive as the simulation process is simpler and the tools available to both measure and predict (model) green water flows can be applied with greater confidence than those for streamflow. There are a number of reasons for this, the most significant being that by focusing on estimates of E, the uncertainties introduced by considering a second dimension (i.e. lateral flow) are avoided. Thus, where accuracy and confidence in the estimates of water use of one crop relative for long term planning is required, the ‘‘green water’’ approach does offer some advantages. However, as highlighted above, long term means of estimated Green Water Flows applied to large spatial scales fall short as management tools. People and ecosystems react at small spatial scales and to short term events such as intra seasonal dry periods. This is particularly true in semi-arid regions where many water resources management issues, such as planning for environmental flows and human water requirements require an understanding of the impact of land use on the flow regime in its entirety. 4.2. Appropriate water resources management and planning tools A dilemma in highlighting the role of Green Water Flows in IWRM exists around the extent to which the information can be simplified, and yet retain a management as well as an illustrative role. As highlighted above, many studies have provided estimates of long term average green and blue water flows in response to land use (Jewitt et al., 2004; Calder et al., 2005; Falkenmark and Ro¨ckstrom, 2004) and climate change (Schulze et al., 2005) at broad spatial and long temporal scales. Whilst these can be used to illustrate the potential impacts of land use change and focus attention on the potential for the management of land use i.e. Green water management tools, these are rarely useable as tools to address the detail required to ensure that daily downstream flow requirements are met. Where such information is needed, these are integrated within hydrological models with varying degrees of complexity and success. As a first step towards the consideration of downstream impacts of RWH in India, Calder et al. (2005) have produced a ‘‘Quadrant’’ approach where, quadrants represent the potential for land use change to cause Green Water Flows to exceed incoming precipitation and where prescribed minimum blue water flows may not be met (Fig. 3). This approach does provide a useful means of
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Fig. 3. ‘‘Quadrant’’ approach where catchment conditions which can be used to identify green and blue water management options and whether benefits would be derived from further soil water conservation measures (after Calder et al., 2005).
highlighting the link between green and blue water flows, especially in a context where consideration of environmental flows and other downstream users is extremely limited. However, this is based on an assumption that there is a direct causal relationship between Green Water Flows and runoff; an assumption that is only valid at large temporal scales and as such is inadequate as a tool to address the day to day concerns of water resources managers and planners. Flow Duration Curves (FDCs) have long been used in assessing assurance of supply and risk associated with the reticulation and supply of water, particularly where large water storage systems are being considered. FDC’s can be derived from both observed and simulated time series and have been shown to be one of the most informative methods of displaying the entire flow record. However, their use when considering run of river and smaller catchment scale situations has been limited. In South Africa, FDCs have been used to integrate the methodologies to assess, estimate and illustrate environmental flows (Hughes and Hannart, 2003) and have been investigated in assessments of commercial afforestation on streamflow (Scott et al., 2000). Recent publications by Brown et al. (2005) and Lane et al. (2005) have highlighted the usefulness of using FDC when assessing the impact of land use change on the flow regime. The use of FDCs also provides a means of assessing risk. In SA SFRA assessments are historically precautionary in favour of the low flow periods (Gush et al., 2002). However, integration of these periods with estimates of environmental flows using the methods adopted by Hughes and Hannart (2003) provides a far more sophisticated means of assessing a potential impact relative to a required assurance of supply for the downstream users or other users in the spatial unit under consideration. In this approach time series of output from hydrological models can be subjected
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to analysis of the percentage of time that the recommended flows to meet environmental or human needs are equaled or exceeded. In South Africa such human and environmental water requirements are expressed in terms of the Reserve – or the amount of blue water required downstream and are derived from numerous studies at different levels of detail. It is accepted that the Reserve amount is not static. It is not a minimum flow requirement, but rather a volume which varies according time of year and which can be expressed as a relative portion of the flow regime rather than in absolute terms. The combination of these tools provides for a framework for water resources planners in the country by which the impact of Green Water Flows by commercial afforestation in South Africa can be assessed (Hughes, 2006). Within this framework, time series from the ACRU Model, capable of detailed representation of green and
blue water flows, representing the likely impact of commercial afforestation can be plotted on the same graph as FDCs representing a reference or naturalized flow, present day flows and the Reserve as illustrated by Figs. 4 and 5. The y-axis of the FDC represents the percentage of natural flow remaining, and plots of this curve will decline with increasing Green Water Flows until the Reserve threshold is reached. Fig. 5 is typical of these analyses in that the lines first intersect at the low flow periods of the flow regime. The use of the SPATSIM framework (Hughes and Palmer, 2005) for this purpose enables direct links between the Quaternary Scale Reserve Determination Model representing blue water flows and SFRAs (green water flows) to be established. Such analyses can provide an expression of the assurance with which such target flows can achieve and thus provide information which is extremely important in water
Fig. 4. The likely impact of different scenarios can be assessed for different months. The model can then be applied to assess the likely impact of increasing the area of the intended tree type (or other similar analyses). Areas where the future flow and Resrve FDC intersect may be problematic.
Fig. 5. The impact can be assessed for different levels of assurance of supply. Different levels of assurance of supply for months of interest can be selected and assessed.
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resources planning and management (Hughes, 2006). This approach also effectively provides an equivalent to the assurance criteria that are typically used to quantify the reliability of a component of a water supply project. Thus in integrating Green Water Flows with blue water management approaches, time series derived from SEBAL or similar remotely sensed or model derived estimates of Green Water Flows can be used as input or components of integrated hydrological modelling systems which in turn can provide FDC’s for assessment of land use impacts on blue water users, including the Reserve, for the required assurance of supply.
5. Discussion/conclusions Green Water Flows should be more explicitly considered in water resources planning and management. However, as highlighted by Falkenmark and Ro¨ckstrom (2005) when interacting with water resources policy makers and planners, we are often restricted to a ‘‘sanctioned discourse’’ which is focused on how existing blue water resources can best be utilized or reallocated and in which there is very little consideration of the broader hydrological cycle. Such a discourse is in danger of perpetuating poor planning, particularly if reallocation of water needs are not more closely linked to land allocation. Although this may seem obvious, these are often separated in policy. A focus on green water, although potentially acceptable for illustrative purposes cannot be used to manage impacts on blue water unless targets or thresholds are set with unrealistically high safety margins. This is required to minimize risks associated with uncertainties regarding the links between green and blue water flows and the generation of flows, particularly low flows in rivers. The challenge to scientists is to provide a means of assessing Green Water Flows in a context that is useable to policy makers. Thus, the use of innovative approaches such the estimations of Green Water Flows from remote sensing which provides a means of understanding spatial and temporal distribution of Green Water Flows and ultimately a means of testing the E component of hydrological models should be encouraged and how this information is presented to policy makers should be refined. Linkages between this information and blue water flows needs to be made, and the use of flow duration curves is a useful means of linking with ‘‘traditional’’ water resources management and planning approaches. Acknowledgements This paper draws from ongoing research which is supported by SIDA, IWMI and IHE through the SSI Programme and the Water Research Commission of South Africa through Project K5/1428.
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