Stage: Water Quality and Flushing Requirements of Lake IJsselmeer in Lelystad

How much water is needed to keep a freshwater lake healthy and maintain good water quality in a changing climate? During this internship, you will investigate flushing requirements of large freshwater lakes and explore what they mean for the future management of Lake IJsselmeer, one of the Netherlands' most important freshwater reservoirs. Your research will contribute to long-term decisions about water availability, water quality and investments in water infrastructure. Interested in combining scientific research with real-world water management challenges? Keep reading to learn more.

Please note: This is a research internship with a strong focus on literature review and analysis. The internship runs for approximately three months, from May to August 2027, and combines scientific research with its practical application to water management at Rijkswaterstaat.

Background

Climate change will increase evaporation rates, increase the length of dry periods, and raise freshwater demand for agriculture, drinking water supply, industry, and nature conservation in the Netherlands.

As a result, less freshwater may remain available for flushing Lake IJsselmeer as well as the Wadden Sea. Major investments are planned over the coming decades in discharge capacity at the Afsluitdijk (pumps and discharge sluices) to maintain the required water levels (safety, water availability and water quality). To support long-term decision making, a better understanding is needed of the minimum flushing requirements of Lake IJsselmeer and the associated operational conditions for the future water system.

Objective
This study will identify reported flushing requirements for large freshwater lakes and estuarine systems, with a primary focus on freshwater lakes, and assess their relevance for the future management of Lake IJsselmeer.
The central question is twofold:

Which flushing requirements are reported for Freshwater Lakes and (to a lesser extent) Estuarine Systems, required to sustain or reach the desired water quality.

What relevance may be found in (some of) those requirements for the future management of Lake IJsselmeer?

Part 1: International Literature Review (approximately two-thirds of the effort)
A structured review of international scientific and technical literature will be conducted on flushing needs (residence time, water renewal, etc.) to reach or maintain good water-quality in large freshwater lakes, in different continents and settings. To a lesser extent estuarine systems may also be considered.
The review will focus on:

Terminology, definitions, and information sources (literature as in Science Journals, Web-based info, etc.) for flushing requirements, water residence time, renewal time etc.

Methods used to determine or approximate minimum flushing requirements for water quality.

Indicators which are used to deternine flushing requirements and/or appropriate water residence time.

Relationships between flushing rates and (specific) water quality (targets), e.g. salinity control, nutrient accumulation / eutrophication, harmful algal blooms, oxygen conditions, ecological functioning.

Examples from managed lake systems where water allocation competes with environmental and water-quality objectives.

Lessons learned regarding climate change impacts and reduced freshwater availability.

The review will additionally provide some information on:

Part 2: Implications for Lake IJsselmeer (approximately one-third of the effort)

The findings from the literature review will be evaluated on transferability to the IJsselmeer region: Are the flushing requirements, from different lakes in different settings, applicable to Lake IJsselmeer? One or two findings which are sufficiently comparable will be translated to the specific context of Lake IJsselmeer and -to a lesser extent- to Lake Markermeer. The analysis will consider:

A basic analysis of the hydrological functioning of these lakes regarding residence time (lake average, local variety).

Indicative residence-time and flushing requirements relevant for maintaining water quality and ecological conditions. Different water quality standards will necessitate different flushing needs.

Potential consequences of reduced freshwater availability due to higher evaporation and increasing water withdrawals.

Possible complications for the realization of specific goals for nature conservation/nature development.

Possible implications for long-term operation and sizing of discharge infrastructure at the Afsluitdijk.

Preliminary boundary conditions and knowledge gaps for future water management strategies.

The translated findings will be briefly evaluated with respect to:

Deliverables
The study will deliver:

A concise synthesis report of international knowledge on water-quality based flushing requirements for (primarily) freshwater lakes and (to a lesser extent) estuarine systems.

A concise description of the hydrological functioning, residence times/refreshment of Lake IJsselmeer and Lake Markermeer.

An assessment of the relevance of the (literature) flushing knowledge for Lake IJsselmeer and to a lesser extent, lake Markermeer.

Based on the deliverables 1, 2 and 3: a set of indicative hydrological and operational boundary conditions that can support future decisions on water allocation and investments in discharge and pumping capacity at the Afsluitdijk.