El teorema de la aplicaci´on de Riemann

RINGAROOMA

7.1 Introduction

The climate drives the hydrological cycle (Crosbie et al. 2010) and projected changes in spatial and temporal patterns of the climate variables will impact on regional hydrological processes, in particular changes in rainfall will be amplified as an impact on surface runoff (Chiew 2006; Wang et al. 2010). Rainfall is the largest factor in the water balance of catchment runoff and flows into rivers and farm dams (Mpelasoka & Chiew 2008). Higher temperatures increase potential evapotranspiration which may further lead to a reduction in surface runoff and also soil moisture levels (Chiew & McMahon 2002; Crosbie et al. 2010).

Surface runoff is an important but variable component of the hydrological balance of a grazed pasture system (Murphy et al. 2004). Runoff is influenced by a range of rainfall characteristics such as amount of rainfall, seasonal distribution and rainfall intensity. Surface runoff is also influenced by soil type and pasture conditions. Ground cover, which is defined as any non-soil material influences both the frequency and the magnitude of runoff events. During a rainfall event, both the soil water content and the available storage capacity of the soil profile influence the water infiltration rate and as a direct result the generation of surface runoff (Murphy et al. 2004). The relationship between the volume of water that reaches water catchments and rainfall is non-linear since it is reliant on soil moisture levels and the implication regarding water balances is that a given reduction in rainfall leads to greater reduction in the water flowing into catchments (Murphy & Timbal 2008; Chiew et al. 2010; Wang et al. 2010).

Large multi-decadal changes in rainfall have been observed across Australia since the 1950‟s (Gallant et al. 2007). A significant impact of rainfall decline, correlated with increases in temperature in south eastern Australia has been a reduction in runoff in water catchments (Hennessy 2007; Chiew et al. 2010). Since the mid 1970‟s in Tasmania, a decline has been observed in mean annual statewide rainfall. This decline has been particularly noticeable since the mid 1990s (Bennett et al. 2010; Bureau of Meteorology 2011). The decline in rainfall has resulted in an estimated 7% decline in surface runoff for the period 1997 to 2007, compared to the long term average (1924 to 2007) (Viney et al. 2009). However, Bennett et al. (2010) as part of the CFT project, projected runoff and river flows to the end of the 21st century and reported that the observed recent decline of statewide runoff is not projected to

continue throughout the 21st century, consistent with the absence of any significant trend in the projected statewide rainfall (Bennett et al. 2010; Grose et al. 2010).

Regional variation in the amount of surface runoff is projected under future climate scenarios. In the central highlands of Tasmania, mean annual surface runoff is projected to significantly decrease by up to 30%, while in the eastern regions mean annual surface runoff is projected to increase. In addition to annual runoff changes, modelling of 78 river flows across Tasmania has indicated that on average 28 of the 78 rivers modelled are projected to have decreased mean annual flows by 2100 (Bennett et al. 2010).

In Tasmania, approximately 67% of dairy farms utilise some form of irrigation, with approximately 30% of dairy land irrigated (Australian Bureau of Statistics 2010b). Irrigation provides the opportunity for more intensive pasture production resulting in higher and more reliable yields. The Tasmanian dairy industry is the major consumer of irrigation water using approximately 38% of the total water, on approximately 23% of the total area of irrigated land (Australian Bureau of Statistics 2010b). Irrigation water for the dairy industry is primarily sourced from surface water, which includes rivers and streams (33%) and on-farm storage dams (49%). A small percentage of irrigation water in the southern region of the state is sourced from government irrigation schemes (e.g. Coal river Valley irrigation scheme) as well as limited access to groundwater supplies (Australian Bureau of Statistics 2010b). Profitability of the Tasmanian dairy industry is in part dependant on water availability and water security. Climatic conditions affect both the availability of water for irrigation and the need to irrigate in order to supplement rainfall. The availability and security of irrigation water for the Tasmania dairy industry is a major issue, with the increasing demand from the dairy industry occurring at a time when irrigation water availability in many river catchments is in a state of decline, along with increasing conflicts between competing agricultural, urban and environmental demands for water.

To assess the potential influences of the projected climate change on surface runoff across the Tasmanian dairying regions, Bennett et al. (2010) simulated surface runoff using the hydrological model SIMHYD (Chiew et al. 2002) for each of the six dairy regions for the period 1971 to 2100 using daily climate data projections generated from the CFT project.

The objectives of this chapter were to:

1. Quantify the projected regional climate change impact on surface runoff from the six bias-adjusted dynamically downscaled GCMs for the six sites over the period 1971 to 2100.

2. Undertake an on-farm case study analysis at Ringarooma, quantifying surface runoff and river flow projections on irrigation requirements using the six bias- adjusted dynamically downscaled GCMs for the period 1971 to 2100.

7.2 Materials and methods

The daily SIMHYD (Chiew et al. 2002) surface runoff data generated by Bennett et al. (2010) as part of the CFT project was generated on a 0.05° grid, calibrated to interpolated observations from the SILO Data Drill 0.05° gridded data source (Jeffrey et al. 2001). Previously in this report, rainfall data from the AWAP 0.05° data source (Jones et al. 2009), interpolated to a 0.1° grid (AWAP 0.1) had been used as the basis for rainfall calibrations. However, the hydrological model SIMHYD is calibrated to interpolated observations from the SILO Data Drill 0.05° gridded data source (Jeffrey et al. 2001). SIMHYD is configured to accept 0.05° gridded data, accordingly, the CFT GCM climate simulations were re-gridded from a 0.1° grid down to a 0.05° grid using a cubic spline interpolation. Further details of the SIMHYD daily runoff data modelling methods are described in Bennett et al. (2010).

Bennett et al. (2010) generated 0.05° gridded bias adjusted daily runoff projections using the ensemble of the dynamically downscaled GCMs of CSIRO-Mk3.5, ECHAM5/MPI-OM, GFDL-CM2.0, GFDL-CM2.1, MIROC3.2 (medres) and UKMO-HADCM3 data for each of the six sites (Section 3.2). The SIMHYD daily runoff data for the period 1st January 1961 to 31st December 2100 was obtained from the TPAC portal (https://dl.tpac.org.au/).

An irrigated dairy farm at Ringarooma was selected as a case study farm to highlight the influences of projected climate change impacts on on-farm water availability. Bennett et al.

(2010) generated Ringarooma river flow projections from the TasSY (Ling et al. 2009), River models using the ensemble of the dynamically downscaled GCMs of CSIRO-Mk3.5, ECHAM5/MPI-OM, GFDL-CM2.0, GFDL-CM2.1, MIROC3.2 (medres) and UKMO- HADCM3 data for the Ringarooma site (Section 3.2). The daily Ringarooma river flow data for the period 1st January 1961 to 31st December 2100 was obtained from the TPAC portal (https://dl.tpac.org.au/). Using the runoff and river flow projections a water budget of the current and projected irrigation requirements for the case study dairy farm was created for the period 1971 to 2100.

The Ringarooma dairy farm is 166 ha in size, with 75 ha (45%) of the farm supported by irrigation infrastructure. Irrigation water is accessed from both an on-farm catchment (dam) and access to river flows (Ringarooma river) (Plate 7.1). The area under irrigation from the on-farm storage dam is 38 ha (51% of the irrigated area) while the area under irrigation from water accessed from the Ringarooma river is 37 ha (49% of the irrigated area).

The on-farm storage dam has a holding capacity of 89 ML and is supported by a surrounding catchment area of 104 ha (Plate 7.1). The farm has a water allocation license to harvest annual surface runoff, however, the water allocation license and amount of harvestable water is determined by two catchment „seasons‟ a „winter season‟ and „summer season‟. The „winter season‟ catchment period runs from May 1st

to November 30th. The farm has a water allocation license to harvest 20% of total surface runoff during the „winter season‟, the remaining 80% is released for downstream users and environmental flows (Van Brecht pers. comm. 2011). The „summer season‟ catchment period runs from December 1st to April 30th. The farm has a water allocation license to harvest 0.34 ML of surface runoff per day for 100 days, resulting in a total allocation of 34 ML during the „summer season‟.

To irrigate the remaining 37 ha water is sourced from the Ringarooma river during the „summer season‟ of December to April. The farm has a water allocation license of 1.125 ML per day for 100 days, in total 112.5 ML of irrigation water is available for the „summer season‟ for this area.

The projected River flows for the Ringarooma river are the flows remaining after all water extractions, diversions and other losses have been accounted for (Bennett et al. 2010). The extractions, diversions and other losses were calculated according to operating guidelines and water licenses that were current as of December 31, 2007. No account has been taken concerning future changes to land use or water management practices that could affect the river flow. The projected river flow under the future climate scenarios presented in this chapter are the changes caused only by climate.