- Research Letter
- Open Access
Variations in soil moisture and their impact on land–air interactions during a 6-month drought period in Taiwan
© The Author(s) 2018
- Received: 3 May 2018
- Accepted: 9 October 2018
- Published: 25 October 2018
This work is a follow-up study to Lin and Cheng (J Hydrometeorol 17:1337–1355, 2016). In our previous study, the Weather Research and Forecasting model was applied to investigate the impact of soil moisture initialization and soil texture on land–air interactions for a short-term 1-month period, in which two typhoons hit Taiwan and the atmospheric condition were wet. In this study, we extend the simulation period to 6 months and target a drought period. The simulation period is from October 1, 2014 to March 31, 2015. During this study period, a lack of rain caused the drought and strict water rationing was enforced in Taiwan. The study objectives are (1) to understand the effect of soil moisture initialization and soil texture on land–air interactions during the 6-month drought period and (2) to identify the distinction between the previous study, where the atmospheric condition was wet, and the current 6-month drought period. Compared to the previous 1-month simulation, the land–air interactions are strengthened in the 6-month drought period, showing the enhanced impact of soil moisture variations on the surface heat flux, air temperature, and local circulation. In addition, the evapotranspiration process is strengthened in this study, indicating that the land–air interactions are significant when the atmospheric condition is dry. A soil moisture-limited evapotranspiration regime was identified in the previous 1-month wet period study, with the soil moisture strongly constraining the evapotranspiration. However, in this study, the evapotranspiration process can be independent of the soil moisture content, once the soil moisture is lower than the wilting point.
- Soil moisture variations
- Land–air interaction
Soil water content is an important parameter that affects water and energy cycles and the land–air interactions. It modulates the partition of total available energy into sensible heat fluxes (SHF) and latent heat fluxes (LHF) at the land surface, and influences the near-surface atmospheric conditions (Seuffert et al. 2002; Hung et al. 2014). LeMone et al. (2007) analyzed data from a flux tower, radar wind profiler and aircraft from the May–June 2002 International H2O Project (IHOP_2002) and the April–May 1997 Cooperative Atmosphere Surface Exchange Study (CASES-97). They found that soil moisture affects the relative magnitude of SHF and LHF.
Various studies have indicated that the interaction between the land surface and the atmosphere is believed to be strong during drought periods and there is also a strong soil moisture–precipitation feedback loop. Zaitchik et al. (2012) indicated that positive soil moisture–precipitation feedbacks can intensify heat and prolong drought under the conditions of a precipitation deficit. In Meng and Shen (2014), the observational evidence indicates that soil moisture had an influence on the hot extremes and the daily temperature range in eastern China. It was also suggested that the previous drier surface conditions might intensify summer hot extremes and could potentially be used to predict extreme heat waves.
Soil moisture is an important source of atmospheric water vapor through the evapotranspiration (ET) process, including plant transpiration and bare soil evaporation. ET is a major component of the continental water cycle, as it returns as much as 60% of the water back to the atmosphere (Oki and Kanae 2006).
In Koster et al. (2004) and Seneviratne et al. (2006), two main ET regimes are defined: a soil moisture-limited regime and an energy-limited regime. In the energy-limited ET regime, when the soil moisture is above a given critical soil moisture value, ET is independent of the soil moisture content. Below the critical soil moisture, the soil moisture constrains the ET (soil moisture-limited ET regime). When the soil moisture is lower than the wilting point (θwilt), no further ET takes place. Seneviratne et al. (2010) defined a climate dry/wet regime, where the soil moisture is higher than the critical soil moisture value and lower than θwilt, and soil moisture does not impact ET. When the soil moisture is between θwilt and the critical soil moisture value, the soil moisture strongly constrains ET and precipitation could be affected by soil moisture anomalies. The critical soil moisture level typically lies between θwilt and the field capacity, and is typically equal to ca. 50–80% of field capacity. Brubaker et al. (1993) indicated that the precipitation of a region could be supplied by ET from the land surface, indicating the importance of land-surface processes in the water balance. Savenije (1995) analyzed moisture recycling over the Sahel region and found that more than 90% of rainfall is from the recycling of moisture through ET. Wythers et al. (1999) proposed that evaporation is related to changes in the resistance to evaporation in a particular soil type, the amount of energy available to drive the evaporative process and the amount of water available to evaporate.
The mean rainfall per year in Taiwan is ~ 2500 mm (CWB 1991); however, most of the rainfall is rapidly lost to the ocean due to the steep mountains in Taiwan. In addition, the rainfall in Taiwan tends to cause zonal or seasonal drought because of the uneven season rainfall distribution through most of the island (the northeastern part of Taiwan is an exception). The major source of rainfall is concentrated during the Mei-Yu season (May–June) and the typhoon season (July to October). On average, 90% of the annual precipitation falls in the wet season (May–October), with the remaining 10% in the dry season (October–April) (Yu et al. 2002). Yu et al. (2006) concluded that rainfall in northern and eastern Taiwan increased on various time scales, but in central and southern Taiwan, it decreased. However, only during the dry season in central Taiwan and the typhoon season in southern Taiwan was the variation significant.
In our previous study (Lin and Cheng, 2016), the Weather Research and Forecasting (WRF) model was applied to investigate the impact of soil moisture initialization and soil texture on land–air interactions for a short-term 1-month period, in which two typhoons hit Taiwan. The current work is a follow-up study to Lin and Cheng (2016). Here, we extend the simulation time to a 6-month drought period.
The purpose of this study is to investigate the impact of soil moisture variations on the land-surface hydrologic process, particularly focusing on a drought period in Taiwan. The study period is from October 1, 2014 to March 31, 2015. During this study period, a lack of rain caused the drought, resulting in strict water rationing being enforced in Taiwan. A detailed description of the episode is discussed in the following section. The objectives are (1) to understand the effect of soil moisture initialization and soil texture on land–air interactions during the 6-month drought period and (2) to identify the distinction between the previous study, where the atmospheric condition was wet, and the current 6-month drought period.
The study period is from October 1, 2014 to March 31, 2015. During this study period, a lack of rain caused the drought and the water supply situation became urgent as Taiwan received the lowest rainfall since 1947. In response, the government in Taiwan made a two-phase water-rationing plan for the water shortage and water rationing was enforced from January 2015 until May 2015 when the Mei-Yu season begins to bring rain.
In our previous study (Lin and Cheng 2016), the WRF meteorological model with the Noah land-surface model was applied to investigate the impact of soil moisture initialization and soil texture on land–air interactions for a short-term 1-month period in Taiwan. Instead of using the National Centers for Environmental Prediction (NCEP) Final (FNL) Operational Global Analysis data, soil moisture from the Global Land Data Assimilation System (GLDAS; Rodell et al., 2004) was utilized to provide the soil moisture initialization process. In addition, updated soil textures based on field surveys (Leung and Chen 1957) in Taiwan were adopted for WRF model. The previous study of a 1-month simulation focused on the short-term weather events and sub-seasonal time scale, which included two typhoon-induced precipitation episodes and a series of clear-sky days (Lin and Cheng, 2016). In this study, we extended the simulation period to 6 months and discussed the soil moisture variations and their impact on the land–air exchange process, particularly focusing on a drought period.
Three WRF sensitivities were conducted. The first simulation was conducted without any update (WRF-base), for which the initial soil moisture was provided by NCEP FNL data. In the second WRF sensitivity test, the soil moisture from GLDAS was utilized to provide the soil moisture initialization process (WRF-GLDAS). The third WRF sensitivity test was applied with GLDAS and updated soil textures (WRF-GSOIL).
At Chiayi site, the WRF-base shows a gradual drying process towards the end of the simulation period, while the variation of soil moisture in WRF-GLDAS and WRF-GSOIL is not apparent due to the dry soil condition. The observed soil moisture at 10-cm depth shows a very low soil moisture in a range less than 20% most of the time except for the rainfall days. All the models overestimated the top-layer soil moisture at Chiayi site. WRF-GLDAS and WRF-GSOIL perform better due to improved initial soil moisture condition. Because Chiayi site is located nearby the rice paddies, the observed soil moisture at the second, third, and fourth soil layers is high and reaches a saturated condition at 150-cm depth. All the models fail to capture the damp soil conditions in deep layers of soil column at Chiayi site that could be due to the improper treatment of the irrigation effects and the interaction between soil layers and the groundwater. For example, the Noah land-surface model only considers a free-drainage lower boundary condition in the bottom soil layer and has no upward groundwater flow into the lowest soil layer (Barlage et al. 2015).
At Hengchun site, all the models overestimated the soil moisture due to too wet initial soil moisture conditions. The gradual drying process throughout the simulation period can be seen from all the simulation results. The soil moisture converges to a similar value towards the end of the simulation period in WRF-base and WRF-GLDAS run at Hengchun site; however, at Chiayi site, due to distinct soil moisture initialization between WRF-base and WRF-GLDAS, the difference of soil moisture is still apparent at Chiayi site even after the 6-month simulation time. Orth and Seneviratne (2012) also indicated that the extremely dry or wet states of the soil tend to increase soil moisture memory.
The soil moisture is a crucial component that affects the land-surface energy and water budgets. It constrains the ET process, which is an important source of water vapor over land, and in turn impacts precipitation. Moreover, soil moisture is involved in a number of feedbacks through the land–air interactions and land-surface hydrologic cycle.
In our previous study (Lin and Cheng, 2016), we investigated the impact of soil moisture initialization and soil texture on land–air interactions for a short-term 1-month period, in which two typhoons hit Taiwan and the atmospheric condition was wet. The simulation results reveal the importance of soil moisture on the land–air interactions, and showed that the initialization of soil moisture and soil texture can affect the near-surface meteorological variables. The simulation results indicate a soil moisture-limited ET regime, in which the soil moisture constrains the ET.
In this study, we extend the simulation period to 6 months and target a drought period. During the 6-month study period, the atmospheric condition is dry due to the lack of the rain. Compared to the study of Lin and Cheng (2016), the ET process is strengthened in this study, indicating that the land–air interactions are significant and ET can be an important source of water vapor when the atmospheric condition is dry. A comparison between the WRF-GLDAS and WRF-GSOIL simulations shows that the soil moisture is reduced in WRF-GSOIL in western Taiwan due to the larger size soil particle that has less capability to hold water, which in turn reduces the soil moisture content compared to the WRF-GLDAS result. The soil moisture is lower in western Taiwan from the WRF-GSOIL, but the LHF is increased, which indicates that ET is independent of the soil moisture content. In fact, due to the unusual dry condition in this study period, the soil moisture can be lower than θwilt. The supply of θwilt according to the soil look-up table generally becomes lower in the updated soil type; hence, the ET process is still on-going in the WRF-GSOIL simulation, while it can cease in the WRF-GLDAS run once the soil moisture is lower than θwilt.
Finally, this study highlights the importance of soil moisture variations for the land–air interactions. In particular, when the atmospheric condition is dry, the soil moisture content and the ET process become significant in the role of the land-surface hydrologic cycle.
Both authors read and approved the final manuscript.
This study was conducted under the research project entitled ‘Development of high resolution atmospheric modeling and long-term atmospheric data sets in Taiwan and East Asia’, supported by the Ministry of Science and Technology, under Grant Number MOST 105-2621-M-008-001. We would like to thank the National Center for High-Performance Computing (NCHC) of Taiwan for providing computational resources and storage resources. We also thank the Central Weather Bureau in Taiwan for providing the surface station data sets and Professor Ben-Jei Tsuang at National Chung-Hsing University for providing the observed soil moisture data sets.
The authors declare that they have no competing interests.
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