Vulnerability of New Zealand pastoral farming to the impacts of future climate change on the soil water regime
Authors: Anthony Fowler, Simon Aiken, Kim Maree
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Executive summary
It is now widely agreed that some of the most significant impacts of future climate change are likely to be associated with the hydrological cycle, including potentially significant impacts on the soil water regime. The importance of agriculture to the New Zealand economy means that any such impacts are important to us. Moreover, the most recent climate change scenarios developed for the New Zealand region suggest possible enhanced vulnerability in some regions, due to likely changes in the strength of prevailing winds, interacting with New Zealand’s complex topography.
The orthodox approach to assessing potential climate change impacts is a “top-down” methodology, which uses a cascade of climate and biophysical models to assess impacts (that is, climate → water resources → agricultural productivity). This methodology is limited by the fact that regional-scale uncertainties about climate change are very large, to the extent that realistic representation of them can result in even the direction of change being uncertain, or reversing when climate change scenarios are revised.
An alternative “bottom-up” approach is proposed here which focuses on the sensitivity of the soil water regime and associated pasture productivity to climate change – in effect addressing the “do we have a potential problem?” question prior to launching a complex (and expensive) full-scale climate change impact assessment. The research is a pilot study with three specific research aims: a) develop a generic methodology for preliminary assessment of the vulnerability of pastoral production to climate change; b) build a user-friendly software tool to implement that methodology, and; c) test both in a case study context (proof of concept).
Core to the methodology used here is the idea that the sensitivity of the soil water regime and pasture productivity to future climate change is best assessed within the context of natural climate variability. To facilitate this:
- Multi-decadal climate time series were used to drive a daily water balance model (DWBM) of near surface hydrology.
- Pasture productivity was calculated from modelled evaporation (excluding interception).
- The input time series were systematically perturbed to assess the sensitivity of soil dryness and pasture productivity to climate change.
- Mean sensitivity of soil dryness and pasture productivity were displayed as two dimensional “response surfaces”.
- Climate change impact assessment was undertaken by superimposing simplified scenarios of future climate change onto these response surfaces.
The significance of the potential impacts was then assessed by comparing the simulated impacts with inter-annual variability (caused by natural climate variability).
Much of the above methodology was implemented in the DWBM and the method and model were tested on a Hawkes Bay data set. That application highlighted the elegance of the underlying concepts and showed that useful results could be obtained very quickly. It was also useful in identifying several implementation issues.
It is concluded that the proposed “bottom-up” methodology is appropriate. The methodology, and the specific DWBM implementation, can reasonably be used to assess the sensitivity of pasture production to climate change. Extension of the analysis to all New Zealand climate regions is recommended, following some specific refinements:
- Use of more sophisticated climate change scenarios that realistically envelop plausible future climates.
- Further development of the DWBM to simplify the end-user experience and reduce the potential for error.
- Explicit testing and refinement of the pasture production model.
- Integration of the pasture production model into the DWBM, to circumvent time consuming and error-prone manual steps.
- Integration into the DWBM of several other manual steps in the methodology.
Contact for Enquiries
Sustainable Land Management and Climate Change
MAF
Pastoral House
25 The Terrace
PO Box 2526, Wellington
Tel: 0800 CLIMATE (254 628)
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