- Significant decreases in water flows are predicted in Sorraia basin through the 21st century, due to reduced precipitation, irrigation practices, high evapotranspiration, reduced soil water content and increased temperatures.
- Water availability for irrigation is projected to decrease in the Sorraia basin by 2060, most drastically in the west and south of the basin.
- Climate models project an increase in both precipitation and runoff around 2030, compared to the present day, and a subsequent decrease around 2060.
- If more efficient fertiliser and irrigation practices are adopted, agricultural practices are projected to potentially be ecologically sustainable in the basin, even under significant climate changes.
Water management issues
Of the 122 water bodies in the Sorraia basin, 54 (44%) are classified as ‘good’ ecological status, 15 (12%) as moderate, 12 (10%) as poor, 2 (c.2%) as bad, and 39 (32%) as unclassified. The key pressures in the Sorraia basin are hydromorphological and hydrological alterations, water abstraction and diffuse pollution (largely driven by agricultural land use) alongside discharges of waste water from urban areas. Demands for water are high in the Sorraia basin, particularly for agricultural irrigation. Urban and agricultural waste water treatment systems are largely inefficient or absent across the basin. The most common multiple stressor combinations in the basin are thus between hydrological (abstraction, flow regulation, construction of dams) and nutrient (point and diffuse pollution) stressors. The three key ecosystem services provided by the Sorraia are water for agricultural irrigation, recreational services (including tourism and angling) and waste water treatment.
Environmental storylines and scenarios
The MARS study in the Sorraia basin investigated the single and combined effects of hydrological and nutrient stressors on several biotic state variables and ecosystem services. The study modelled different scenarios of future climate, land use and management changes to evaluate the ecological effects of multiple stressors. The MARS scenarios forecast environmental changes in the context of three storylines describing future political, economic, social and climatic changes, modelled for ten year periods around both 2030 (to support the WFD) and 2060 (to show the impacts of climate change). Each scenario incorporates a different climate model based on Representative Concentration Pathways 4.5 and 8.5.
Each scenario was tailored to the Sorraia basin through projected changes to land use and water demands, which were developed through expert judgement and local stakeholder engagement.
In Techno World, economic growth is prioritised and environmental management is typically ad hoc, and focused on instrumental gains such as flood and drought protection. Ongoing human development of the landscape is expected with this storyline, although some increase in efficiency might compensate environmental deterioration.
Consensus World simulates economic growth and environmental protections similar to present day. Expected changes are minimal compared with the other story lines and are based on current trends.
Fragmented World frames a global situation where countries have little regard for environmental protection. This was considered the worst case scenario, with an overall increase of environmental degradation.
For each scenario, four possible River Basin Management Plan measures were simulated:
- No water regulation: irrigation reservoirs are no longer used;
- Efficient agricultural irrigation and fertilisation practices;
- A combination of both measure 1) and 2);
- Afforestation of the catchment, with no irrigation reservoirs or agricultural land use.
Results and conclusions
Findings of scenario analysis
Future scenarios predict a significant decrease in water flows due to the combined effects of reduced precipitation, ongoing irrigation practices, high evapotranspiration, reduced soil water content and increased temperatures. The choice of model influenced results: GFDL modelled lower water flows, whilst IPSL projected increased nutrient loads. Projected future nitrogen and phosphorous loads for 2030 and 2060 are increased by agricultural expansions and increases in fertiliser use in the Techno and Fragmented scenarios. Sediment loads – a proxy for soil erosion and phosphorous transport – projections are variable depending on the climate model used, decreasing in each storyline for the GDFL model, but increasing in the Techno and Fragmented scenarios for the IPSL model.
Water availability for irrigation is projected to decrease in the Sorraia basin by 2060, particularly under the Techno scenario. In all scenarios, the lowest water availability is projected to be in the west and south of the basin. The ability of the basin to purify nutrient loads is projected to reduce in the Techno and Fragmented scenarios by 2060, and stay relatively close to present day levels in the Consensus scenario. Ecological status – as shown by fish-based metrics and ecological quality indicators – is affected positively by altitude, and negatively by urban and agricultural areas in the upstream catchment. Future changes to ecological status are projected to only be minor, with slight reductions in good and very good status sites in the Techno and Fragmented scenarios.
Significance for water management
Proposed management Measures 2 (efficient agricultural and irrigation practices) and 3 (as 2, with no water regulation in reservoirs) caused river flows to be maintained through the year in 2060. This finding suggests that agricultural practices can be sustainable in the basin if water use is efficient, even under significant climate changes. Proposed Measure 4 (afforestation) projects a significant increase in evapotranspiration in the basin, which reduces river flows. This finding suggests that the ecosystem benefits of significant afforestation (e.g. riparian buffers) should be weighed up against potential decreases in river flows.
Measures involving increased fertiliser efficiency (2 and 3) and afforestation (4) reduced nutrient loads in the Sorraia by 2060. Based on projected changes to fish populations, Measure 1 (no water regulation) was the least effective strategy for conserving ecological status. Measures 2 and 3 were effective at conserving ecological status under all scenarios for the IPSL model, and for the Techno and Fragmented scenarios for the GFDL model.
Overall, Measures 2 (efficient agricultural and irrigation practices) and 3 (as 2, with no water regulation in reservoirs) were the most effective at maintaining river flows and conserving ecological status.
The Sorraia basin in central Portugal has an area of 7730 Km2 and a length of 155km. The Sorraia river joins the larger Tagus river near the town of Murteira. The basin has been an important agricultural area for centuries due to its fertile soils. The Sorraia Valley Irrigation Plan involves the use of two reservoirs – Montargil and Maranhão – in the upper catchment supplying water along irrigation channels to agricultural land, and influencing river discharge. Around half of the basin is agricultural land (largely arable cropland and pasture), which forms the largest artificially irrigated area in Portugal. The other half of the basin is dominated by cork and oak forests. The basin supports 153,099 human inhabitants, largely concentrated in three towns: Ponte de Sôr, Samora Correia and Coruche. The basin has a Mediterranean climate with hot, dry summers and colder, wet winters.
Context for modelling
The study used a linked set of process-based, empirical and climate models. SWAT is a process-based watershed model, which was used to model basin hydrology (runoff, precipitation, evaporation, infiltration and lateral flow) in response to land use. The outputs of the SWAT model fed into empirical models (boosted regression trees and random forests) which modelled the response of biological indicators to multiple stressors, and in turn gave indications of projected ecosystem status and service provision. The SWAT model was calibrated and validated using data from 1996-2015. Two climate models were used: GFDL-ESM2M and IPSL-CM5A-LR. The models contribute to a DPSIR (drivers, pressures, state, impact, response) model for the Sorraia basin, in which the effects of different management interventions on ecosystem status and services (including irrigation water, drinking water, water purification, recreation and tourism) is conceptualised.
Materials and methods
Data for the SWAT model was sourced from the Shuttle Radar Topography MIssion (topography), the SROA (soil types, Serviço de Reconhecimento e Ordenamento Agrário), GSE Land and Global Cover (land use) and SNIRH (Sistema Nacional de informação de Recursos hídricos) databases (weather). Two datasets were used in empirical modelling: an EU EFI+ dataset (sampled 1995-2005) was used to assess the response of fish-based functional indicators to multiple stressors; and a Portuguese Environmental Agency dataset from WFD monitoring (2010-11) was used to assess the response of biotic status indicators. The two datasets comprised 23 predictor variables, including 8 land use pressure variables (agriculture, cropland, forest, urban; in primary and upper catchment); 2 nutrient stressors (phosphorous and nitrogen), 7 hydrological stressors (relating to river flow dynamics) and 6 variables describing natural environmental variability (including weather, altitude, slope, distance from source and catchment size).
Facts and Figures: SORRAIA
MARS (2017) Case study Sorraia- Case Study Synthesis - Deliverable 4.1. (Download report, 26.1mb)