# Surface Water Temperature Predictions at a Mid-Latitude Reservoir under Long-Term Climate Change Impacts Using a Deep Neural Network Coupled with a Transfer Learning Approach

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## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

#### 2.1. Data Acquisition

#### 2.1.1. Target Site and Observed Data

^{2}. It is approximately 4 km long, 0.5 km wide, and 80 m deep when it is at total water capacity (112,000,000 m

^{3}), and has a large flood surface area (4.2 km

^{2}). We obtained the monthly meteorological data and surface water temperature (–1–0 m) from 1984 to 2020, which was spontaneously measured between 10:00 and 15:00. The surface water temperature was recorded as approximately 0 $\mathbb{C}$(e.g., 0.5 $\mathbb{C}$) when ice cover appeared during winter. The number of data points was approximately 450. We assumed that the data could be representative of typical values in each month. Figure 2a shows the temporal variations in air and surface water temperatures observed near the levee of the Tokachi Dam reservoir. Scattered plots between both datasets indicate a moderate relationship with a linear regression of the coefficient of determination (R

^{2}), as shown in Figure 2b. Note that only air temperature is available as onsite weather data for about a 35-year observation period.

#### 2.1.2. General Circulation Model and Weather Research and Forecast Model Data

#### 2.2. Long Short-Term Memory Model

#### 2.3. Transfer Learning (TL) Approach

#### 2.4. Evaluation

#### 2.5. Procedures and Setups of the Computation

- air temperature strongly affects the surface water temperature;
- the humid subarctic climate in the Tokachi River watershed (the target) changes into a humid subtropical climate at Kyushu (the source). Note that the TL approach possibly adjusts the source climate to the target climate, although past air temperature at the source was significantly higher than that at the target;
- the effect of water level variations is involved in the variations of surface water temperature;
- no sediment accumulation affects the topographical aspects of the reservoir;
- no geological changes occur in the surrounding environments;
- the effect of the presence or absence of ice cover is included in the values of the surface water temperature;
- the effect of the air−water interaction on surface water temperature is homogeneous among lakes.

## 3. Results

^{2}and the values of RMSE and NSE. Excluding Case 0, strong relationships were evident with R

^{2}> 0.8 and NSE > 0.9. The prediction in Case 2 was marginally better than that in Case 1, with a 10% reduction in RMSE because of the additional input of the air−temperature difference within a time step. The prediction in Case 3 was marginally worse than that in Case 2, with a 7% increase in RMSE because of the marginal effect of inherent features of surface water temperatures at Kyushu. However, Case 3 was able to predict higher surface water temperatures in over 18 $\mathbb{C}$ than Case 2 (Figure 7c,d). This feature suggests that Case 3 can be beneficial for a future prediction under the global warming impacts.

## 4. Discussion

^{2}and NSE (refer to Figure 7). While comparing the accuracy with other studies, our RMSEs (1.8–2.0 $\mathbb{C}$) were comparable to the outputs from a single DNN, as used in past studies [9,28], although their computational setups were different from ours in terms of the amount of data and the target of water temperature profiles. Therefore, our LSTM model may be quantitatively reliable when considering the availability of only a few reports in lake and reservoir environment studies.

## 5. Conclusions

- The LSTM model that was validated with the observed data achieved accurate reproducibility calculations with R
^{2}> 0.8 and NSE > 0.9. In particular, Case 2 with two input datasets, i.e., air temperature and difference in air temperature, was marginally better than the other cases. - Past and future predictions with locally downscaled data showed that the LSTM model with the TL approach (Case 3 model) was more realistic for future prediction than that without the TL approach based on the difference between past and future air temperatures. The Case 3 model suggested that the frequency ratios with respect to the predicted surface water temperature were increased in the highest range of water temperature and decreased in the lowest range, because the model predicted higher water temperatures.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## Appendix A

^{2}(0.63). The characteristics of the 18 reservoirs are listed in Table A1.

**Figure A1.**Observed data for air temperature ($\mathbb{C}$) and surface water temperature ($\mathbb{C}$) at Kyushu: (

**a**) temporal variations connected with an annual cycle between different datasets, and (

**b**) the relationship between air and surface water temperatures with a linear regression line (R

^{2}= 0.63).

Name | Location | Total Capacity (m^{3}) | Surface Area (km ^{2}) | Catchment Area (km ^{2}) |
---|---|---|---|---|

Ayakita | 32.0975° N, 131.1422° E | 21,300,000 | 0.82 | 149.3 |

Ayanann | 32.0578° N, 131.1217° E | 38,000,000 | 1.36 | 101.0 |

Dokawa | 32.3553° N, 131.3453° E | 33,900,000 | 1.54 | 143.0 |

Hikawa | 32.5714° N, 130.7865° E | 6,300,000 | 0.35 | 57.4 |

Hirowatari | 31.7167° N, 131.2675° E | 6,400,000 | 0.38 | 34.4 |

Houri | 32.7158° N, 131.5736° E | 5,774,000 | 0.28 | 45.2 |

Ichifusa | 32.3200° N, 131.0128° E | 40,200,000 | 1.65 | 157.8 |

Iwase | 31.9428° N, 131.1403° E | 57,000,000 | 4.13 | 354.0 |

Kawabe | 31.4450° N, 130.4456° E | 2,920,000 | 0.23 | 30.2 |

Matsuo | 32.2839° N, 131.3714° E | 45,202,000 | 1.95 | 304.1 |

Midorikawa | 32.6273° N, 130.9089° E | 46,000,000 | 1.81 | 359.0 |

Hase-miyazaki | 32.1458° N, 131.3403° E | 2,250,000 | 0.14 | 11.8 |

Nichinann | 31.6369° N, 131.2758° E | 6,000,000 | 0.41 | 59.2 |

Okita | 32.5506° N, 131.6192° E | 2,750,000 | 0.27 | 8.8 |

Tachibana | 32.1322° N, 131.2700° E | 10,000,000 | 0.29 | 70.5 |

Tashirobae | 32.1367° N, 131.1197° E | 19,270,000 | 1.02 | 131.5 |

Turuta (old) | 31.9853° N, 130.4958° E | 123,000,000 | 3.61 | 805.0 |

Urita | 31.9267° N, 131.3086° E | 720,000 | 0.07 | 4.4 |

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**Figure 1.**Maps of the target area (T), the Tokachi River watershed, and a target reservoir, i.e., the Tokachi Dam reservoir, where (O) is Obihiro, one of the meteorological stations of the Japan Meteorological Agency (JMA), and (S) is a source area at Kyushu.

**Figure 2.**Observed data for air temperature ($\mathbb{C}$) and surface water temperature ($\mathbb{C}$) at Tokachi Dam reservoir: (

**a**) temporal variations and (

**b**) the relationship between air and surface water temperatures with a linear regression line (R

^{2}= 0.75).

**Figure 3.**Air temperature values in Weather Research and Forecasting (WRF) model data: (

**a**) data reliability of past WRF data when compared with the observed data at the Obihiro meteorological station recorded by JMA, including a linear regression line of R

^{2}(= 0.89); (

**b**) variations in the frequency ratios of past and future WRF data and the frequency difference (future data subtracted by past data) with respect to air temperature with a 1$\mathbb{C}$ interval.

**Figure 4.**Sketch of layer structures: (

**a**) long short-term memory (LSTM); (

**b**) transfer learning (TL) approach. Activ. func. = activation function (i.e., Activ. func. 1 = hyperbolic tangent; Activ. func. 2 = sigmoid).

**Figure 6.**Temporal variations during the period of May 1984–July 2020 of calculated and observed surface water temperatures ($\mathbb{C}$). In the legend in the figure, Obs. represents observation; moreover, the case names are as listed in Table 2.

**Figure 7.**LSTM model outputs with the observed data in surface water temperature for four cases: (

**a**) Case 0 as a reference (a linear regression), (

**b**) Case 1, (

**c**) Case 2, and (

**d**) Case 3, showing a relation between Prd and Obs data with a linear regression line of R

^{2}. Dotted lines indicate zero Prd data. The root mean square error (RMSE) and Nash–Sutcliffe efficiency coefficient (NSE) are defined in Table 1. Prd = reproducibility calculation and Obs = observation.

**Figure 8.**Comparisons between the past and the three future predictions in the Case 2 model for surface water temperature. Variations in the frequency ratios of the past and three future (near, mid, and far) predictions and the three differences (future data subtracted by past data) with respect to surface water temperature with 2$\mathbb{C}$ intervals.

**Figure 9.**Comparisons between the past prediction in Case 2 and the three future predictions in the Case 3 model for surface water temperature, showing frequency ratios of the past and three future (near, mid, and far) predictions and the three differences (future data subtracted by past data) with respect to surface water temperature with 2$\mathbb{C}$ intervals.

Hyperparameters and Function | Values or Equations | Remarks |
---|---|---|

Number of LSTM layers | 1 | |

Number of nodes | 20 | |

Past and present time in input | $-$6 to 0 | Time interval = month |

Lead time in output | 1 | Time interval = month |

Batch size | 100 | |

Number of epochs | 1000 | Retaining the TL approach and has the same number |

Learning rate | 0.01 | |

Dropout rate | 0.0 | |

Reproducibility | None | |

Optimizer | Stochastic gradient descent (SGD) | |

Activation function | Sigmoid Hyperbolic tangent | Range from 0 to 1 Range from −1 to 1 |

Loss function | Sum of squared residuals = $\frac{1}{2}{\displaystyle \sum}_{i=1}^{N1}{\left({V}_{ci}-{V}_{oi}\right)}^{2}$ | ci = model calculation, oi = observed data, N1 = the number of data |

Error evaluation functions | RMSE =$\sqrt{\frac{1}{N1}{\displaystyle \sum}_{i=1}^{N1}{\left({V}_{ci}-{V}_{oi}\right)}^{2}}$ | Same as above |

NSE =$1-{\displaystyle \sum}_{i=1}^{N1}{\left({V}_{ci}-{V}_{oi}\right)}^{2}/{\displaystyle \sum}_{i=1}^{N1}{\left({V}_{oi}-{V}_{o}\right)}^{2}$ | Same as above, and <*> = average |

Name | Model | Input Data | Transfer Learning |
---|---|---|---|

Case 0 | Linear regression | Air temperature | No |

Case 1 | LSTM | Air temperature | No |

Case 2 | LSTM | Air temperature, difference in air temperature | No |

Case 3 | LSTM | Same as above | Yes |

Result | Pre-Trained Model Applied to the Past WRF Data | Pre-Trained Model Applied to the Future WRF data | * Net Heat-Related Factor (Ratio °C) | ||
---|---|---|---|---|---|

Near Future | Mid Future | Far Future | |||

Figure 8 | Case 2 | Case 2 | −0.27 | −0.23 | 0.04 |

Figure 9 | Case 2 | Case 3 | 0.03 | 0.06 | 0.06 |

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**MDPI and ACS Style**

Kimura, N.; Ishida, K.; Baba, D. Surface Water Temperature Predictions at a Mid-Latitude Reservoir under Long-Term Climate Change Impacts Using a Deep Neural Network Coupled with a Transfer Learning Approach. *Water* **2021**, *13*, 1109.
https://doi.org/10.3390/w13081109

**AMA Style**

Kimura N, Ishida K, Baba D. Surface Water Temperature Predictions at a Mid-Latitude Reservoir under Long-Term Climate Change Impacts Using a Deep Neural Network Coupled with a Transfer Learning Approach. *Water*. 2021; 13(8):1109.
https://doi.org/10.3390/w13081109

**Chicago/Turabian Style**

Kimura, Nobuaki, Kei Ishida, and Daichi Baba. 2021. "Surface Water Temperature Predictions at a Mid-Latitude Reservoir under Long-Term Climate Change Impacts Using a Deep Neural Network Coupled with a Transfer Learning Approach" *Water* 13, no. 8: 1109.
https://doi.org/10.3390/w13081109