10:45 〜 12:15
[AHW19-P05] Trends in winter and spring runoff in Niigata Prefecture under a changing climate
キーワード：snowmelt runoff, snowmelt timing, climate change, water resources, Niigata Prefecture
Niigata Prefecture experiences some of the heaviest snowfall in Japan, producing large mountain snowpack which melts during the spring and early summer giving much needed water resources for agriculture. Niigata is one of the most important rice-producing regions, and it relies on water from melting snow for the irrigation of paddy rice. However, recent climate change means that on average the winters are becoming warmer with less snowfall, and this is already affecting the timing and amount of spring snowmelt runoff in the rivers of Niigata Prefecture. In this study, we analyse the impacts of warming temperatures on the seasonal timing of river runoff, using two different indices of snowmelt timing. In particular, we evaluate how the timing of spring snowmelt runoff has changed in recent decades, and examine the relationship between the timing of spring snowmelt runoff and mean temperatures during the snow cover season.
For the Uono River basin (1408 km2, period of record 1960-2021), a major tributary of the Shinano River in central Niigata Prefecture, we found that the timing of snowmelt runoff is strongly related to mean temperatures during December to April (R2 = 0.87 and 0.80). From a linear regression model, we see that an increase in the mean temperature (DJFMA) of 1 degree Celsius is associated with the snowmelt timing occurring 7 to 9 days earlier, depending on the index of snowmelt timing adopted. Similar regression models were developed for the Takiya River basin (19.5 km2, period of record 2000-2022), a tributary of the Miomote River in northern Niigata Prefecture, using mean temperatures during January to March (R2 = 0.75 and 0.74). An increase in mean temperature (JFM) of 1 degree Celsius was also associated with snowmelt timing occurring 7 to 9 days earlier.
Using the non-parametric Mann-Kendall trend test, we identified statistically significant trends (p-value <0.05) related to climate and seasonal timing of runoff in Niigata Prefecture. For the Uono River, there was a significant warming trend for winter and spring temperatures, indices of snowmelt timing became significantly earlier, winter to early spring runoff (DJFM) increased with very high significance (p = 0.0001), while the main snowmelt season of April (p = 0.002) and April-May (p = 0.02) showed highly significant decreasing trends. For the downstream Shinano River basin (9719 km2, POR 1960-2021) a similar trend was found, with increases in winter to early spring runoff (DJFM, p = 0.005), while the decrease in April runoff and shift in snowmelt timing was not significant. We can understand that warming temperatures are causing winter precipitation to shift from snowfall to rainfall (decreasing snow fraction), leading to increased winter runoff and reduced snowpack and earlier/reduced snowmelt runoff.
The recent winter of 2020 produced record warm temperatures and record snow scarcity throughout much of Japan. Indeed, at Takiya River the snowmelt timing was the earliest ever recorded by a margin of about 10 days, and April (snowmelt) runoff was a record low. Taken as a whole, these results show significant trends for increasing winter runoff and earlier/reduced spring snowmelt runoff due to the progression of climate change in Niigata region. This is having a negative impact on water resources availability for rice paddy agriculture in the region.
For the Uono River basin (1408 km2, period of record 1960-2021), a major tributary of the Shinano River in central Niigata Prefecture, we found that the timing of snowmelt runoff is strongly related to mean temperatures during December to April (R2 = 0.87 and 0.80). From a linear regression model, we see that an increase in the mean temperature (DJFMA) of 1 degree Celsius is associated with the snowmelt timing occurring 7 to 9 days earlier, depending on the index of snowmelt timing adopted. Similar regression models were developed for the Takiya River basin (19.5 km2, period of record 2000-2022), a tributary of the Miomote River in northern Niigata Prefecture, using mean temperatures during January to March (R2 = 0.75 and 0.74). An increase in mean temperature (JFM) of 1 degree Celsius was also associated with snowmelt timing occurring 7 to 9 days earlier.
Using the non-parametric Mann-Kendall trend test, we identified statistically significant trends (p-value <0.05) related to climate and seasonal timing of runoff in Niigata Prefecture. For the Uono River, there was a significant warming trend for winter and spring temperatures, indices of snowmelt timing became significantly earlier, winter to early spring runoff (DJFM) increased with very high significance (p = 0.0001), while the main snowmelt season of April (p = 0.002) and April-May (p = 0.02) showed highly significant decreasing trends. For the downstream Shinano River basin (9719 km2, POR 1960-2021) a similar trend was found, with increases in winter to early spring runoff (DJFM, p = 0.005), while the decrease in April runoff and shift in snowmelt timing was not significant. We can understand that warming temperatures are causing winter precipitation to shift from snowfall to rainfall (decreasing snow fraction), leading to increased winter runoff and reduced snowpack and earlier/reduced snowmelt runoff.
The recent winter of 2020 produced record warm temperatures and record snow scarcity throughout much of Japan. Indeed, at Takiya River the snowmelt timing was the earliest ever recorded by a margin of about 10 days, and April (snowmelt) runoff was a record low. Taken as a whole, these results show significant trends for increasing winter runoff and earlier/reduced spring snowmelt runoff due to the progression of climate change in Niigata region. This is having a negative impact on water resources availability for rice paddy agriculture in the region.