Graham Jewitt

Complexity, Modeling, and Natural Resource Management

This paper contends that natural resource management (NRM) issues are, by their very nature, complex and that both scientists and managers in this broad field will benefit from a theoretical understanding of complex systems. It starts off by presenting the core features of a view of complexity that not only deals with the limits to our understanding, but also points toward a responsible and motivating position. Everything we do involves explicit or implicit modeling, and as we can never have comprehensive access to any complex system, we need to be aware both of what we leave out as we model and of the implications of the choice of our modeling framework. One vantage point is never sufficient, as complexity necessarily implies that multiple (independent) conceptualizations are needed to engage the system adequately. We use two South African cases as examples of complex systems—restricting the case narratives mainly to the biophysical domain associated with NRM issues— that make the point that even the behavior of the biophysical subsystems themselves are already complex. From the insights into complex systems discussed in the first part of the paper and the lessons emerging from the way these cases have been dealt with in reality, we extract five interrelated generic principles for practicing science and management in complex NRM environments. These principles are then further elucidated using four further South African case studies—organized as two contrasting pairs—and now focusing on the more difficult organizational and social side, comparing the human organizational endeavors in managing such systems.

Using Participatory Scenario Planning to Identify Ecosystem Services in Changing Landscapes

There is a growing interest in assessing ecosystem services to improve ecosystem management in landscapes containing a mix of different ecosystems. While methodologies for assessing ecosystem servi ...

Forest and Water Policies. The need to reconcile public and science perceptions

This paper compares and contrasts some of the science and public perceptions of the role of forests in relation to the water environment. It is suggested that the disparity between the two perceptions needs to be addressed before we are in a position to devise and develop land and water policies (whether market or non-market based) which are aimed at either improving the water environment, and by doing so improving the livelihoods of poor people by greater access to water, or conserving and protecting forests. Examples are given of three research projects in South Africa, India and Costa Rica where, through the involvement of stakeholder groups, often with representatives comprising both the science and public perceptions, interactive research programmes were designed not only to derive new research findings with regard to the biophysical processes but also to achieve better “ownership” and acceptance of research findings by the stakeholders. It is concluded that to move towards a reconciliation of the different perceptions and to put in place better policies and management systems, where policy is better connected with science, will require further efforts: a) To understand how the “belief” systems underlying the science and public perceptions have evolved, and how these are affecting land and water policy processes; b) To develop management support tools, ranging from simple dissemination tools, which can demonstrate the impacts of land use decisions on the water environment to institutions and local people, to detailed robust and defensible hydrological models which are needed to help implement the new land and water policies, such as those now being implemented in RSA; and c) To understand better how land and water related policies impact on the poorest in society. It is argued that many present policies may not be significantly benefiting the poor and may even, in some situations, be resulting in perverse outcomes.

Annual water, sediment, nutrient, and organic carbon fluxes in river basins: A global meta‐analysis as a function of scale

Process controls on water, sediment, nutrient, and organic carbon exports from the landscape through runoff are not fully understood. This paper provides analyses from 446 sites worldwide to evaluate the impact of environmental factors (MAP and MAT: mean annual precipitation and temperature; CLAY and BD: soil clay content and bulk density; S: slope gradient; LU: land use) on annual exports (RC: runoff coefficients; SL: sediment loads; TOCL: organic carbon losses; TNL: nitrogen losses; TPL: phosphorus losses) from different spatial scales. RC was found to increase, on average, from 18% at local scale (in headwaters), 25% at microcatchment and subcatchment scale (midreaches) to 41% at catchment scale (lower reaches of river basins) in response to multiple factors. SL increased from microplots (468 g m−2 yr−1) to plots (901 g m−2 yr−1), accompanied by decreasing TOCL and TNL. Climate was a major control masking the effects of other factors. For example, RC, SL, TOCL, TNL, and TPL tended to increase with MAP at all spatial scales. These variables, however, decreased with MAT. The impact of CLAY, BD, LU, and S on erosion variables was largely confined to the hillslope scale, where RC, SL, and TOCL decreased with CLAY, while TNL and TPL increased. The results contribute to better understanding of water, nutrient, and carbon cycles in terrestrial ecosystems and should inform river basin modeling and ecosystem management. The important role of spatial climate variability points to a need for comparative research in specific environments at nested spatiotemporal scales.

Improving Crop Yield and Water Productivity by Ecological Sanitation and Water Harvesting in South Africa

This study quantifies the potential effects of a set of technologies to address water and fertility constraints in rain-fed smallholder agriculture in South Africa, namely in situ water harvesting (WH), external WH, and ecological sanitation (Ecosan, fertilization with human urine). We used the Soil and Water Assessment Tool to model spatiotemporally differentiated effects on maize yield, river flow, evaporation, and transpiration. Ecosan met some of the plant nitrogen demands, which significantly increased maize yields by 12% and transpiration by 2% on average across South Africa. In situ and external WH did not significantly affect the yield, transpiration or river flow on the South Africa scale. However, external WH more than doubled the yields for specific seasons and locations. WH particularly increased the lowest yields. Significant water and nutrient demands remained even with WH and Ecosan management. Additional fertility enhancements raised the yield levels but also the yield variability, whereas soil moisture enhancements improved the yield stability. Hence, coupled policies addressing both constraints will likely be most effective for improving food security.

Modelling highly variable daily maximum water temperatures in a perennial South African river system

Thirty-three months of observed hourly water temperatures were used to calculate daily maximum water temperatures for nine sites within the Sabie-Sand River system, Mpumalanga Province, South Africa. A suite of statistical models for simulating daily maximum water temperatures, of differing complexity and using inputs of air temperature, flow rates, rainfall and relative humidity, were developed and verified. Whilst all models performed well, the most suitable was a site-specific multiple linear regression model using inputs of mean daily air temperature, minimum daily air temperature and relative humidity. The inclusion of a flow rate term would greatly enhance the utility value of such models, but insufficient flow rate data is often a limiting factor. For pragmatic purposes, a simple non-linear regression model using mean daily air temperatures is probably adequate for many areas of South Africa. A generic statistical water temperature model at a daily time step is difficult to achieve, whereas catchment- or site-specific, models, were found to be more appropriate.

Hydrograph separation using tracers and digital filters to quantify runoff components in a semi-arid mesoscale catchment

1 IHE, Delft, The Netherlands University of KwaZulu‐Natal, Centre for Water Resources Research, Pietermaritzburg, South Africa Section of Water Resources, Delft University of Technology, Delft, The Netherlands UN World Water Assessment Programme (WWAP), UNESCO, Perugia, Italy Umgeni Water Chair of Water Resources Management, University of KwaZulu‐Natal, Pietermaritzburg, South Africa Correspondence Aline Maraci Lopes Saraiva Okello, IHE, Delft, The Netherlands. Email: a.saraiva@un‐ihe.org

Assessing suitability of the ACRU hydrological model in a rainforest catchment in Ghana, West Africa

Abstract Hydrological modelling is a challenge in the rainforest dominated Bonsa catchment (1482 km2) in Ghana, West Africa, because of data scarcity and rapidly changing land uses. The objective of this study was to assess the suitability of the daily time step physical-conceptual ACRU model for hydrological modelling in the Bonsa catchment. Since the catchment is data poor, model calibration was conducted using a careful parameterization and sensitivity analysis, using initial values obtained from literature and field observations, as well as climate data for the period 1987–1999 and 1991 land use. The model performance during calibration and validation was satisfactory, with a monthly NSE of 0.6 and 0.5 and R2 of 0.8 and 0.7, respectively. The model simulated the rise and the recession of the hydrograph well, but during the validation the accumulated monthly streamflows were underestimated by 10%. The main conclusion from this study is that the ACRU hydrological model is suitable for exploring basic hydrological responses to land use and climate in the Bonsa and similar catchments.

A comparison of productive and non-productive green water-use efficiency of Podocarpus henkelii and Pinus patula in the KwaZulu-Natal Midlands

A number of studies undertaken in South Africa to quantify the green water-use (total evaporation) of introduced commercial forestry species have shown conclusively that green water-use from commercial forest plantations is substantially higher than from the original grasslands or fynbos that were replaced by afforestation. Green water can be categorised into productive (transpiration) and non-productive (canopy and litter interception and soil evaporation) fluxes. There is a widespread perception within South Africa that indigenous tree species, in contrast to commercial forestry genera/species, are water-wise and should thus be planted more extensively in view of their more efficient use of water. However, information on the water-use of indigenous trees and forests is scarce and indirect, and the relative contributions of transpiration, canopy interception and litter interception to total evaporation have until now not been investigated in South Africa. To quantify these fluxes, both field measurements and modelling were undertaken. In this study, green water-use by indigenous Podocarpus henkelii and an exotic species, Pinus patula, were compared. The results from this study showed that the productive green water-use by P. henkelii and P. patula was 41.0% and 95.9% of gross precipitation, respectively, over the 18-month period of this study. The non-productive canopy and litter interception by P. henkelii accounted for 29.8% and 6.2%, respectively, while canopy and litter interception accounted for 22.1% and 10.7%, respectively, for P. patula. The productive green water-use efficiency (WUE) of P. henkelli and P. patula is 7.14 g mm−1 and 25.21 g mm−1, respectively, in comparison with the total green WUE of 3.8 g mm−1 and 18.8 g mm−1. From a water resources management and planning perspective it is important to consider the total green WUE, but also to have a good understanding of the relative contributions of each component of the green water fluxes so that water abstracted from the soil can be differentiated from the water that does not reach the soil due to losses of canopy and litter interception and does not get lumped as one evaporative loss.