We investigated the origins of rain- and subsurface waters of north-central Namibia's seasonal wetlands, which are critical to the region's water and food security. The region includes the southern part of the Cuvelai system seasonal wetlands (CSSWs) of the Cuvelai Basin, a transboundary river basin covering southern Angola and northern Namibia. We analysed stable water isotopes (SWIs) of hydrogen (HDO) and oxygen (H218O) in rainwater, surface water and shallow groundwater. Rainwater samples were collected during every rainfall event of the rainy season from October 2013 to April 2014. The isotopic ratios of HDO (δD) and oxygen H218O (δ18O) were analysed in each rainwater sample and then used to derive the annual mean value of (δD, δ18O) in precipitation weighted by each rainfall volume. Using delta diagrams (plotting δD vs. δ18O), we showed that the annual mean value was a good indicator for determining the origins of subsurface waters in the CSSWs. To confirm the origins of rainwater and to explain the variations in isotopic ratios, we conducted atmospheric water budget analysis using Tropical Rainfall Measuring Mission (TRMM) multi-satellite precipitation analysis (TMPA) data and ERA-Interim atmospheric reanalysis data. The results showed that around three-fourths of rainwater was derived from recycled water at local–regional scales. Satellite-observed outgoing longwave radiation (OLR) and complementary satellite data from MODerate-resolution Imaging Spectroradiometer (MODIS) and Advanced Microwave Scanning Radiometer (AMSR) series implied that the isotopic ratios in rainwater were affected by evaporation of raindrops falling from convective clouds. Consequently, integrated SWI analysis of rain-, surface and subsurface waters, together with the atmospheric water budget analysis, revealed that shallow groundwater of small wetlands in this region was very likely to be recharged from surface waters originating from local rainfall, which was temporarily pooled in small wetlands. This was also supported by tritium (3H) counting of the current rain- and subsurface waters in the region. We highly recommend that shallow groundwater not be pumped intensively to conserve surface and subsurface waters, both of which are important water resources in the region.
The ratio of latent heat flux to available energy, termed the evaporative fraction (EF), and the ratio of latent heat flux to downward shortwave radiation (ES), are two useful evaporative flux ratios (EFR) for estimating daily evaporation. Both EF and ES remain relatively constant during daytime, but their value varies from day to day. It is yet unclear if long‐term change signals are detectable given the uncertainty associated with the diurnal variations. Using EF and ES data obtained during the major rainy seasons on a tableland of the Loess Plateau of China, we showed that day‐to‐day variability in the EF or ES was detectable given diurnal variation. The EF and ES showed slight increasing trends from midmorning to afternoon, but the ES was superior to the EF for satellite‐based monitoring of long‐term evaporation trends.
Aircraft-based remote sensing was carried out over the Lena River region (approximately 100 km horizontal scale) near Yakutsk, in eastern Siberia, on nine days between 24 April and 19 June 2000. A home-use video camera was installed on the aircraft and aerial images of the land surface were recorded. By flying at very low altitudes (100 m and 150 m), we compensated for the low resolution of the camera. In all, 5515 scenes were sampled at 10-s intervals from the video, visually interpreted, and classified. The snow/ice and water cover conditions were also interpreted. The results showed that grassland covers the riverine lowland (RLL) of the Lena River, while the principal form of land cover over the terraces is larch forest (about 61%). There was a remarkable contrast between the left (LBT) and right (RBT) bank terraces of the Lena River, no-forest areas covered 28.8% of the RBT, whereas no-forest areas covered only 13.8% of the LBT. In addition, the LBT had a greater proportion of birch forest than the RBT (15.2% vs 2.5%). The video data over eight days showed that the snow thawed first in the RLL and last in the forested areas of the LBT and RBT. An increase in the area covered by water in the RLL in mid-May was probably due to flooding by the Lena River. Distinguishing insolated from cloud-shadowed scenes of the land surface revealed that the insolation rate was lower over the LBT and RBT than over the RLL. The land-cover database created in this analysis will play an important role as basic, reliable ground-truth information for studies using satellite images.
Nighttime transpiration in a larch forest in northern Japan was investigated using concurrent measurements of the energy budget below and above the canopy and sap flow velocities. Upward latent heat flux ( lE ) above the canopy was observed on ∼20% of nights during the growing season. Nighttime lE was ∼7% of daily totals during the dry season (September–October) and averaged ∼3% over the entire growing season. A relatively low contribution of the understory to lE (<30%) and strong synchronicity between larch sap flow rates and lE on two warm windy nights indicated nighttime lE , occurring when warm dry air intruded from above, was largely due to transpiration from the larch overstory. High canopy conductance on some nights relative to reported maximum cuticular conductance and a strong correlation between canopy conductance and air humidity on these nights indicate the stomata of the larch trees did not close tightly at night.
A liquid nitrogen freezing method was used to collect raindrops for the determination of isotope-size distribution. Water drops that fall onto a surface of liquid nitrogen stay suspended for 10 to 20 s, until their temperature reaches the Leidenfrost point (126 K). As their temperature falls to the freezing point, they release their heat by thermal conduction. At the freezing point, latent heat of fusion is released, along with a significant loss of water. After freezing completely, the ice droplets stay suspended, cooling by thermal conduction until they reach the Leidenfrost point. They then lose buoyancy and start sinking. Consistent isotopic changes of 1.5 +/- 0.4 and 0.33 +/- 0.05 per thousand for hydrogen and oxygen, respectively, were found for droplets with radii between 1.0 and 1.5 mm. Isotope fractionation appeared to occur at the same time as water loss, as the droplets were freezing, in what was probably a kinetic effect.
This study analyzed controls on evapotranspiration ( E ) from a western Siberian bog, from a perspective that surface constraints limit the E available for atmospheric evapotranspiration demands. Ratios of E to potential evapotranspiration ( E P ) ranged from 0.2 to 0.9, and were clearly related to changes in surface constraints represented by the bulk transfer coefficient for latent heat C E (= β C H where β is the surface moisture availability and C H is the bulk transfer coefficient for sensible heat). Both E P and equilibrium evaporation ( E EQ ) showed similar seasonal trends, suggesting the importance of radiation to the seasonal variation of E P . The atmospheric drying power ( E a ) was a minor factor in E P and showed less seasonal change during most of the growing season. However, the presence of a dry air mass caused by synoptic scale advection (most frequently observed in May 1999) significantly enhanced E a ; consequently, the seasonal maximum of E P occurred earlier than the seasonal maximum of E EQ . Values for C H (0.004–0.011) increased with leaf area index except Sphagnum moss (LAI g ), indicating that vegetation growth contributes to changes in bog roughness through canopy height changes. The β value gradually decreased with decreases in the water table position ( z wt ); the open water surface area and water content of Sphagnum moss depended on z wt . Furthermore, the absence of a significant relationship between β and phenology implies that changes in evaporation contribute to variations in E more than changes in transpiration. Hence roughness change created by vegetation growth and surface wetness limit evapotranspiration to less than the potential evapotranspiration.
This study explores how north-south displacements of the Baiu front influence isotopic variation in Baiu precipitation. Deuterated water vapor (HDO), which was measured continuously in central Japan during the rainy season in 2010, showed clear intraseasonal variations associated with north-south migration of the Baiu front. Because water vapor transport by lower tropospheric southwesterlies from the subtropical high is characterized by relatively high HDO/H2O isotopic ratios, the highest HDO/H2O isotopic peaks were observed when the Baiu front migrated to the north of the observation site. Although these data were collected over only a one-year period, the findings can be used to explain interannual isotopic variations in Baiu precipitation during the period 1961-1978 at the Tokyo station. The isotopic content of June precipitation showed a positive correlation with the ratio of warm rainfall formed by subtropical marine air to total rainfall in June. This suggests that the isotopic content of Baiu precipitation is higher than that of a normal year, for years when the Baiu front shows prolonged stationary patterns in regions towards the Sea of Japan. Thus, we conclude that water isotopic ratios are a powerful tool for reconstructing past shifts in the position of the Baiu front.
In the far North of Siberia, air temperature is increasing due to global warming. The warmer-than-normal air temperature thawed permafrost. Flood was caused when a large amount of permafrost thawed. Objectives are to monitor permafrost degradation using microwave remote sensing, to demonstrate spatio-temporal patterns of flood, and to understand flood-generating mechanism in order to manage flood risk. After ice wedge melting due to the increased air temperature, the melting water gushed from the ground. Then, the melting water flowed into a nearby river, leading to floods. Ground subsidence by permafrost degradation was measured using DInSAR of ALOS/PALSAR. Flood damages were quite different by location. In downstream area around Andryushkino, flood damage continued for two years. The flood water flowed slowly toward the north, because landscape in this area is almost flat without slope. Therefore, the local people who lived in Andryushkino suffered flood for a long time.
