What has Soil Got to Do with Water?

Transcription

1 What has Soil Got to Do with Water? Leeper Memorial Lecture for 2003 by Robert E White Institute of Land and Food Resources The University of Melbourne Deficiencies in the debate 1. Too much focus on managing flows in rivers engineering devices 2. Too little focus on the paradox of too much saline water underground, and too little good quality water in the rivers Objectives 1. Introduce basic interaction between soil, vegetation and water, and pathways of water movement 2. Socio-economic analysis on a catchment scale to determine the costs and benefits of land use change to achieve desired biophysical outcomes 3. Show that progress can only be made when we deal holistically with soil-vegetationwater issues, in consultation with communities Overview 1. Objectives 2. Basic interactions between soil, vegetation and water 3. Managed sustainability? 4. A case history Sustainable Grazing Systems 5. An example of a holistical approach to water management in the landscape 6. Conclusions and recommendations 1

2 Soilvegetationwater interactions The hydrologic cycle, defined by the water balance equation Water Balance Equation P = ET + R + D + S P = Rainfall ET = Evapotranspiration R = Surface runoff + subsurface flow D = Deep drainage S = Change in soil water storage Root zone Water pathways in a duplex soil Rainfall Evapotranspiration A horizon - sandy loam Surface runoff Subsurface flow B horizon - clay Pathways of water movement in soil are diverse, and the relative amounts of water moving by individual pathways are highly variable. For example, Deep drainage (potential recharge to groundwater) Surface of a sodic soil after rain 2

3 Runoff (surface and subsurface lateral flow) rapidly contributes to stream flow The rise of saline groundwater due to increased Recharge in the landscape a slow process Nowhere is the problem of soil and water management more acute than in the Murray- Darling Basin (MDB) CEO Don Blackmore argues for managed sustainability 1. Forest clearance Recharge area However, the MDB over the past 200 years represents a paradigm of managed unsustainability 4. Encroaching salinity Discharge area 2. Increased deep drainage 3. Watertable rises 3

4 Runoff in the Murray-Darling Basin Occurrence of salinity in the Murray-Darling Basin in 1996 What are the real numbers? Pre-development, the median annual flow of the Murray-Darling River was 13,900 gigalitres (GL) 1 GL = 1000 megalitres (ML) = 1 billion litres (1 ML = 1 Olympic swimming pool, and 500 GL = 1 Sydney Harbour) Total NSW Vic. 13,900 GL of river discharge from the Murray- Darling catchment of 1.1 million km 2 is equivalent to only 13 mm effective runoff annually (Note, 1 ML water per square km = 1 mm depth of water spread over that area) An increase in average runoff of 2 mm would provide approx. an extra 1 mm, or 1,100 GL water annually 4

5 Sustainable Grazing Systems (SGS) was Meat & Livestock Australia (MLA) program, supported by State agencies and universities, which focused on - more profitable and - more sustainable grazing systems in the high rainfall zone (> 600 mm) of southern Australia SGS involved cooperation between meat and wool producers and resource scientists from A National Experiment (NE) was established with 10 main sites extending from Albany in WA to Tamworth in NSW From the water balance equation P = ET + R + D + S The National Experiment Sites of the SGS program In southern Australia, we find surplus water is generated mainly in winter (P is high and ET very low), so that P ET = (R + D) = surplus water, and D/(R + D) = the partition ratio In the NE, long-term weather data ( ) was used to determine - the probability of a winter surplus of water occurring at individual Sites, and - the average size (in mm) of that surplus Probabilities of surplus water at Carcoar, NSW, based on long term weather data Probability of surplus greater than a given value (%) Flexibly grazed native pasture Continuously grazed native pasture Winter surplus water W w (mm) 5

6 Average winter surplus (mm) Average winter surplus related to the perenniality of the vegetation Vasey, W Vic. Ruffy, NE Vic. Albany, blue gums A soil-water-plant-animal process model (the SGS Pasture Model was used to simulate water losses on - different soil types under - different pastures (perennial vs annual species), and - grazing managements Perenniality (%) Mean annual drainage, (mm) Effect of grass rooting depth on mean annual drainage (over 31 years) on a Yellow Sodosol Phalaris Annual ryegrass Redgrass Kikuyu Partition ratio Effect of soil type on the partitioning of surplus water, PR = drainage/ (drainage + runoff) Albany Carcoar Maindample Vasey Rooting depth (cm) Red Chromosol Brown Sodosol Yellow Sodosol Sandy Sodosol Conclusions Changing from annual species (annual ryegrass) to perennials (phalaris, kikuyu) decreased winter surplus water by 40-50% (39-97 mm) 2-3 trees per hectare Only plantation trees gave complete control of deep drainage and hence control of potential recharge to groundwater Spaced trees (red gums) up to 14 per ha saved 44 mm water relative to phalaris pasture 6

7 Conclusions (cont) Soil type interacted with climate and pasture type to have a major effect on the winter surplus Soil type had a major effect on the partitioning of surplus water between runoff and deep drainage Strategies to manage water in complex landscapes must be holistic, encompassing, economic, social, and environmental goals A holistic approach to better water management First, spatially referenced biophysical data (regional climate, hydrology and salt stores, soil type, soil fertility) must be used to determine optimal land use for specific water management outcomes on public and private land Second, the land use options need to be subjected to a critical economic analysis to determine the costs and benefits of change for both public land (the community) and private landholders Take as an example the Goulburn-Broken (GB) Catchment in Victoria, part of the Murray-Darling Basin Remnant vegetation Irrigated pasture Horticulture Crop-pasture Pasture Rainfall (c.480 mm) 1.82 million (M) ha, of which 0.6 M ha threatened by salinity 0.86 M ha could be revegetated Public land (c.1000 mm) Given the twin aims of decreasing recharge to groundwater that exacerbates dryland salinity, and increasing runoff that supplies good quality water to streams Use a hydrologic model in a GIS framework to assign cropping, dryland pastures, and plantation trees to land areas, taking account of local climate hydrology (and the distribution of salt stores, saline seeps) slope and soil type (soil depth and profile form) soil fertility (e.g. soil ph) pasture species (perennial vs annual) proximity to streams 7

8 Preliminary results suggested that - 88 to 449 GL water could be saved (diverted from groundwater), if 20 to 100% of the available area was revegetated - more water could be diverted to streams if pasture replaced trees in some areas Consider the overall impact if this approach was applied in other MD catchments, BUT The most important aspect was the economic/financial analysis of land use changes to show revenue change costs incurred other gains and losses that can be given a dollar value analysis of extra risk non-monetary considerations Conclusion Only when biophysical modelling is combined with economic modelling can individual landholders know the consequences of change for their businesses, and public authorities know the costs to the community of achieving desirable social and environmental outcomes What about climate change? CSIRO modelling suggests there will be 20 to 130 mm less water available in soil-plant systems in southern Australia by 2030 Therefore, a contingency factor needs to be built into all water supply models, based of catchment water balances Final conclusions and recommendations As much attention should be focused on the controls for generating surplus water in catchments, as on the regulation of river flows Management of water should be on a wholeof-catchment basis, but individual landholders, CMAs etc need to have realistic financial analyses of the consequences of change The social consequences of change for individuals and communities need to be to be accounted for Acknowledgements Colleagues in the Institute of Land and Food Resources and the Australian Society of Soil Science Inc. Colleagues in the Sustainable Grazing Systems program Esme Annette White for support and encouragement 8