Preprints
https://doi.org/10.5194/hess-2021-621
https://doi.org/10.5194/hess-2021-621

  04 Jan 2022

04 Jan 2022

Review status: this preprint is currently under review for the journal HESS.

Seasonal forecasting of lake water quality and algal bloom risk using a continuous Gaussian Bayesian network

Leah Jackson-Blake, François Clayer, Sigrid Haande, James Sample, and Jannicke Moe Leah Jackson-Blake et al.
  • Norwegian Institute for Water Research (NIVA), 0349 Oslo, Norway

Abstract. Freshwater management is challenging, and advance warning that poor water quality was likely, a season ahead, could allow for preventative measures to be put in place. To this end, we developed a Bayesian network (BN) for seasonal lake water quality prediction. BNs have become popular in recent years, but the vast majority are discrete. Here we developed a Gaussian Bayesian network (GBN), a simple class of continuous BN. The aim was to forecast, in spring, total phosphorus (TP), chlorophyll-a (chl-a), cyanobacteria biovolume and water colour for the coming growing season (May–October) in lake Vansjø in southeast Norway. To develop the model, we first identified controls on inter-annual variability in water quality using correlations, scatterplots, regression tree based feature importance analysis and process knowledge. Key predictors identified were lake conditions the previous summer, a TP control on algal variables, a colour-cyanobacteria relationship, and weaker relationships between precipitation and colour and between wind and chl-a. These variables were then included in the GBN and conditional probability densities were fitted using observations (≤ 39 years). GBN predictions had R2 values of 0.37 (cyanobacteria) to 0.75 (colour) and classification errors of 32 % (TP) to 13 % (cyanobacteria). For all but lake colour, including weather nodes did not improve predictive performance (assessed through cross validation). Overall, we found the GBN approach to be well-suited to seasonal water quality forecasting. It was straightforward to produce probabilistic predictions, including the probability of exceeding management-relevant thresholds. The GBN could be purely parameterised using observed data, despite the small dataset. This wasn’t possible using a discrete BN, highlighting a particular advantage of using GBNs when sample sizes are small. Although low interannual variability and high temporal autocorrelation in the study lake meant the GBN performed similarly to a seasonal naïve forecast, we believe the forecasting approach presented could be useful in areas with higher sensitivity to catchment nutrient delivery and seasonal climate, and for forecasting at shorter time scales (e.g. daily to monthly). Despite the parametric constraints of GBNs, their simplicity, together with the relative accessibility of BN software with GBN handling, means they are a good first choice for BN development, particularly when datasets for model training are small.

Leah Jackson-Blake et al.

Status: open (until 01 Mar 2022)

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Leah Jackson-Blake et al.

Leah Jackson-Blake et al.

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Short summary
We developed a Gaussian Bayesian Network (GBN) for seasonal forecasting of lake water quality, including algal bloom risk, in a shallow nutrient-impacted lake in southern Norway. Discrete Bayesian networks are commonly used in environmental modelling, but this is one of very few applications of a continuous BN. The GBN approach proved to be promising, particularly where training datasets are small.