Root-zone storage and snow cover effects

Abstract

Extreme weather events seem to happen more often nowadays. One of these extreme events was the California Drought between 2012 and 2016. The socioeconomic and environmental impacts of this drought were enormous:
thousands square kilometers of agricultural land fallowed, thousands of lost jobs, salinization problems and forest fires. As a result of the drought, multiple research is conducted related to the hydrological and environmental
impacts. However, the role of the root-zone storage capacity during the drought period is not investigated well. This root-zone storage capacity is the water available for plants to transpire and to grow. It influences the partitioning between the transpiration and the run-off rates, which controls the fundamental
processes in ecosystem functioning (such as floods, droughts and groundwater recharge). Scientific evidence is growing that this root-zone storage behaves dynamically. The underlying assumption is that plants adapt their root system to climatic and environmental changes. One of these changes related to drought is the availability of snow, which is an important source of the rivers in California. It is important to know how the rootzone is changing due to the snow cover changes during the California Drought to understand the hydrological
behaviour of the Californian rivers. For that, the Merced River basin in the Sierra Nevada is analyzed on the relationship between snow cover and root-zone storage capacity.
To analyze these relationship, this research consists of two parts: trend analysis and hydrological modeling.
The purpose of the first is to see whether there is a change in snow cover, temperature and precipitation and to learn about the dynamics between precipitation, temperature and snow cover. This is done by conducting a trend analysis with the Mann-Kendall test for precipitation, temperature and snow-free days. These snowfree days are calculated based on snow cover data from a MODIS satellite product (MOD10A1) and a method of regional snow-line elevation. Additional to this, a multivariate regression analysis was executed to related snow-free days, precipitation and temperature to each other. Furthermore, scenarios for snow-free days were conducted to analyze the effect of the CaliforniaDrought. The purpose of the second part is to relate the change
in snow cover to the root-zone storage capacity. This is done by building a hydrological model calibrated on the snow cover data. By doing this, the maximum soil moisture deficit over a 15 year interval could be calculated,
which is an estimation for the root-zone storage capacity.
The trend analysis showed a clear downward trend in winter precipitation, but not a significant trend in temperature. The snow-free days are increasing at all elevations, but only significant up to 3300 m.a.s.l. The maximum trend related to the observed snow-free days was 7 days/year at 2300 m.a.sl. and the average for the whole catchment was 4 days/year. The multivariate regression analysis showed that the temperature influence ismore important than the precipitation input, however the maximum R2 was around 0.7. At higher elevations it was not so clear whether precipitation or temperature described snow-free days well enough. The scenarios re veiled the enormous effects of the California Drought on the total amount of snow-free days and confirmed the importance of temperature during this extreme event. At 2300 m.a.s.l. it differs 40 snow-free days with a situation with normal winter temperatures. The effect of temperature is also visualized in the hydrological model, where the maximum soil moisture deficit was not found in the drought period but slightly before it. This is caused by the combination of small precipitation amounts and a low temperature (no melt). In the drought period the soil moisture deficit was limited by the melt from the snow. This is also confirmed by the analysis of the snow storage in the hydrological model. However, the maximum of the soil moisture deficit is gradually increasing of the past 30 years which hints to the adaptive behaviour of the root-zone.
The trend analysis and the hydrological model showed that the amount of snow in the Merced River basin is decreased due to the California Drought. At 2300 m.a.s.l., the snow-free days are changed from 170 to over 250. The maximumdifference in snow storage before and after 2010 is 100mmaveraged over the whole basin.
The decrease of snow during this period had a limiting effect on the soil moisture deficit. Due to the melting water, no maximum values of soil moisture deficit were found. However, slightly before the California Drought
a maximumwas found. This maximum is related to a cold but dry period, where not a lot of melting water was generated. Overall, the maximum of the soil moisture deficit is increasing over the past 30 years. This hints to the adaptive behaviour of plants. In summary, the California Drought caused a decrease of the amount of snow and resulted in limiting of the soil moisture deficit. Simultaneously the soil moisture deficit is increasing.
The limiting effect of snow melt on the soil moisture deficit gives plants the time to cope with the changes.