Normal Dry Season Research Articles

Impact of 2016 intense drought to small tropical reservoir: An early onset and late recovery experience in Timah Tasoh, Malaysia

This study investigates the hydrological drought impact during the extreme 2016 El-Nino to small tropical reservoir of Timah Tasoh, in Perlis, Malaysia. Small tropical reservoirs are important freshwater supplier due to its economic & strategic factors including (i) closer distance to inhabitants and agriculture footprints, (ii) easy access to the resource, and (iii) cheaper cost of water pipelines system. In theory, small tropical lakes or reservoirs are facing higher risk to drought compared to the bigger ones. However, detail information and evidence are required for site specific drought adaptation in the future especially the onset and offset of the drought at specific hydrometeorological and dam scale. Prior to that, a case study was conducted in one of the small-sized and shallow reservoirs of Timah Tasoh, Perlis to analyze the impact. The larger Pedu reservoir was selected as control site. Both reservoirs were having identical hydro-climatic characteristics (seasonality of rainfall). The observation on the rainfall and reservoir level data indicated that the small sized reservoir (Timah Tasoh) tends to dry about 2 months faster (reached dam critical level) and recover 1 month late compared to the normal dry season. In comparison with the Pedu reservoir, the drought onset and offset in Timah Tasoh was earlier and late respectively although the rainfall pattern is typical. The total impacted drying days for Timah Tasoh is 197 days compared to 78 days of Pedu (about 3 months longer). The findings indicate that the 2016 extreme drought impacted the Timah Tasoh small reservoir where they would suffer longer dry season and experienced slower recovery compared to the bigger Pedu reservoir. The outcome of this investigation signifies that the early drought onset monitoring should be implemented for the small sized reservoirs to take appropriate mitigation and preventive measures in adapting to the intense drought.

Groundwater quality : comparing alluvial plain and tertiary volcanic in tropical region

Sustainable water resources are the most important issue related to the effects of land use change. However, aspects of geomorphology and lithology have not been studied by researchers before. This study aims to analyze the characteristics of groundwater systems associated with lithology and geomorphology. The method used in this study was a survey. Data retrieval is done during the long dry season (tropical region), and the normal dry season in regions with different land forms and lithology. Analyze the data by comparing groundwater samples with physical water quality. The results showed that in tertiary volcano areas there is a decrease in the quality of groundwater physically during the long dry season, while in alluvial plains there is no physical decrease in water quality. The decrease in water quality occurs during the long dry season. The decrease in groundwater quality is influenced by the dry season, the litology of quarland volcanic rocks, and the shape of tertiary volcanic landform.

Hydraulic safety margins of co-occurring woody plants in a tropical karst forest experiencing frequent extreme droughts

Hydraulic safety margins (HSM) have been widely studied to assess the hydraulic risks and/or strategies of plant species under drought. However, calculations of HSM tend to vary between studies, leading to conflicting conclusions. In this study, HSM of 16 co-occurring woody species (including evergreen trees, brevi-deciduous trees, and lianas) in a tropical karst forest was investigated. They were expressed as: (1) the difference between minimum leaf water potential and water potential causing 50% loss of leaf hydraulic conductance (HSMleaf), (2) the difference between water potential at stomatal closure and that at 50% loss of branch hydraulic conductivity (HSMstomatal), and (3) the difference between water potential at 50% loss of leaf hydraulic conductance and that at 50% loss of branch hydraulic conductivity (HSMsegmentation). We asked the following questions: were HSMleaf in the normal and extreme dry seasons across plant groups different? Were there correlations between different HSM-calculations? Results showed that, on average, the three plant groups showed similar and positive HSMleaf in the normal dry season, but evergreen trees declined to a more negative value in the extreme dry season while the other two groups remained positive. The massive loss of leaf hydraulic conductance in several evergreen tree species was consistent with their extensive shoot dieback under extreme droughts. Across species, there were no significant relationships between HSMstomatal and HSMleaf. Most species (mainly lianas and brevi-deciduous trees) showed negative HSMsegmentation, which did not support the vulnerability segmentation hypothesis that branches are more resistant to cavitation than leaves. Moreover, more negative HSMsegmentation tended to have lower HSMstomatal and larger HSMleaf in the extreme dry season. This study indicates that karst evergreen trees are more likely to experience leaf hydraulic failure under extreme droughts, and reveals potential correlations between branch and leaf hydraulic safety strategies. Further studies on HSM-demographic rate relationship in the tropical karst forests are recommended.

Species-Specific Shifts in Diurnal Sap Velocity Dynamics and Hysteretic Behavior of Ecophysiological Variables During the 2015-2016 El Niño Event in the Amazon Forest.

Current climate change scenarios indicate warmer temperatures and the potential for more extreme droughts in the tropics, such that a mechanistic understanding of the water cycle from individual trees to landscapes is needed to adequately predict future changes in forest structure and function. In this study, we contrasted physiological responses of tropical trees during a normal dry season with the extreme dry season due to the 2015–2016 El Niño-Southern Oscillation (ENSO) event. We quantified high resolution temporal dynamics of sap velocity (Vs), stomatal conductance (gs) and leaf water potential (ΨL) of multiple canopy trees, and their correlations with leaf temperature (Tleaf) and environmental conditions [direct solar radiation, air temperature (Tair) and vapor pressure deficit (VPD)]. The experiment leveraged canopy access towers to measure adjacent trees at the ZF2 and Tapajós tropical forest research (near the cities of Manaus and Santarém). The temporal difference between the peak of gs (late morning) and the peak of VPD (early afternoon) is one of the major regulators of sap velocity hysteresis patterns. Sap velocity displayed species-specific diurnal hysteresis patterns reflected by changes in Tleaf. In the morning, Tleaf and sap velocity displayed a sigmoidal relationship. In the afternoon, stomatal conductance declined as Tleaf approached a daily peak, allowing ΨL to begin recovery, while sap velocity declined with an exponential relationship with Tleaf. In Manaus, hysteresis indices of the variables Tleaf-Tair and ΨL-Tleaf were calculated for different species and a significant difference (p < 0.01, α = 0.05) was observed when the 2015 dry season (ENSO period) was compared with the 2017 dry season (“control scenario”). In some days during the 2015 ENSO event, Tleaf approached 40°C for all studied species and the differences between Tleaf and Tair reached as high at 8°C (average difference: 1.65 ± 1.07°C). Generally, Tleaf was higher than Tair during the middle morning to early afternoon, and lower than Tair during the early morning, late afternoon and night. Our results support the hypothesis that partial stomatal closure allows for a recovery in ΨL during the afternoon period giving an observed counterclockwise hysteresis pattern between ΨL and Tleaf.

English

Water is the principal resource for all living to prosper but a major limiting factor when mismanaged. The wide spread clearing of trees has caused soil erosion that is resulting silt accumulation in the reservoirs 40 to 50% /year. Purposive selection techniques have deployed for this study to gather information.&nbsp; Borana community has been managing water and pasture using its customary institutions called &#39;Abbaa Gadaa&#39; which formulates and enforces general laws. Open surface water are the major water sources for study area. Accordingly the respondents (80%) runoff water, during wet season while ponds, boreholes and micro-dams during the dry season. More than 81% respondents explain that community near the water point involved in almost all stages of water work activities. The mobility of herd from village is determined by water and/or pastures availability and seasonal variation; travel short distance in wet and normal dry season but long distance during drought season as per 79% of respondents. This long distance mobility is termed as the &#39;Furaa&#39; herd movement. The water point development conducted in the past has negative impact on traditional systems in managing and usage of the resources. This is because of lack that proper land use planning and insufficient resource to effectively maintain and manage water points by local government.&nbsp; This paper also reveals that policy makers should oversee any interventions in water case as per integration of science and indigenous knowledge in order to sustain the solution to scarcity of water in the area Key words: Water resources, community, indigenous knowledge, water points. &nbsp

Causes of reduced leaf-level photosynthesis during strong El Niño drought in a Central Amazon forest.

Sustained drought and concomitant high temperature may reduce photosynthesis and cause tree mortality. Possible causes of reduced photosynthesis include stomatal closure and biochemical inhibition, but their relative roles are unknown in Amazon trees during strong drought events. We assessed the effects of the recent (2015) strong El Niño drought on leaf-level photosynthesis of Central Amazon trees via these two mechanisms. Through four seasons of 2015, we measured leaf gas exchange, chlorophyll a fluorescence parameters, chlorophyll concentration, and nutrient content in leaves of 57 upper canopy and understory trees of a lowland terra firme forest on well-drained infertile oxisol. Photosynthesis decreased 28% in the upper canopy and 17% in understory trees during the extreme dry season of 2015, relative to other 2015 seasons and was also lower than the climatically normal dry season of the following non-El Niño year. Photosynthesis reduction under extreme drought and high temperature in the 2015 dry season was related only to stomatal closure in both upper canopy and understory trees, and not to chlorophyll a fluorescence parameters, chlorophyll, or leaf nutrient concentration. The distinction is important because stomatal closure is a transient regulatory response that can reverse when water becomes available, whereas the other responses reflect more permanent changes or damage to the photosynthetic apparatus. Photosynthesis decrease due to stomatal closure during the 2015 extreme dry season was followed 2months later by an increase in photosynthesis as rains returned, indicating a margin of resilience to one-off extreme climatic events in Amazonian forests.

Enhanced canopy growth precedes senescence in 2005 and 2010 Amazonian droughts

Unprecedented droughts hit southern Amazonia in 2005 and 2010, causing a sharp increase in tree mortality and carbon loss. To better predict the rainforest's response to future droughts, it is necessary to understand its behavior during past events. Satellite observations provide a practical source of continuous observations of Amazonian forest. Here we used a passive microwave-based vegetation water content record (i.e., vegetation optical depth, VOD), together with multiple hydrometeorological observations as well as conventional satellite vegetation measures, to investigate the rainforest canopy dynamics during the 2005 and 2010 droughts. During the onset of droughts in the wet-to-dry season (May–July) of both years, we found large-scale positive anomalies in VOD, leaf area index (LAI) and enhanced vegetation index (EVI) over the southern Amazonia. These observations are very likely caused by enhanced canopy growth. Concurrent below-average rainfall and above-average radiation during the wet-to-dry season can be interpreted as an early arrival of normal dry season conditions, leading to enhanced new leaf development and ecosystem photosynthesis, as supported by field observations. Our results suggest that further rainfall deficit into the subsequent dry season caused water and heat stress during the peak of 2005 and 2010 droughts (August–October) that exceeded the tolerance limits of the rainforest, leading to widespread negative VOD anomalies over the southern Amazonia. Significant VOD anomalies were observed mainly over the western part in 2005 and mainly over central and eastern parts in 2010. The total area with significant negative VOD anomalies was comparable between these two drought years, though the average magnitude of significant negative VOD anomalies was greater in 2005. This finding broadly agrees with the field observations indicating that the reduction in biomass carbon uptake was stronger in 2005 than 2010. The enhanced canopy growth preceding drought-induced senescence should be taken into account when interpreting the ecological impacts of Amazonian droughts.

Is fog an important water source for woody plants in an Asian tropical karst forest during the dry season?

AbstractFog is an important supplemental source of water in regions with heavy fog and little rainfall. In the present study, we used the stable isotope content of xylem water to investigate the dry season water source partition between soil and fog water of two evergreen tree species (Cleistanthus sumatranus and Lasiococca comberi), two deciduous tree species (Mayodendron igneum and Lagerstroemia tomentosa), two liana species (Acacia pennata and Combretum latifolium) and understory seedlings of three tree species (C. sumatranus, L. comberi and Celtis philippensis) in a tropical karst forest in southern Yunnan, China. The soil gravimetric water content (GWC) and volumetric water content (VWC), pre‐dawn leaf water potential (Ψpre‐dawn) and leaf water absorbance were also measured. There were no significant differences in both GWC and VWC among 10 to 100 cm depths in the dry season. In the dry season 2009, Ψpre‐dawn of the two evergreen tree species and the understory seedlings down to −4.5 to −6.3 MPa, and which were significantly lower than those of a normal dry season. The proportion that fog contributed to xylem water ranged from 15.8% (understory seedling of C. sumatranus) to 41.3% (liana species C. latifolium). After immersing in water for 3 h, leaves of all the studied species absorbed a significant volume of water. Our results reveal that the woody plants in the tropical karst forest of Xishuangbanna use fog water as an important supplement in the dry season, with more fog water being used by lianas than by trees. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Remote Sensing Vegetation Hydrological States Using Passive Microwave Measurements

A novel technique that links vegetation properties and ET fluxes with a microwave "emissivity difference vegetation index" (EDVI) has been developed and applied to the Amazon region. These EDVI values can be derived from a combination of satellite microwave measurements with visible and infrared observations. This technique is applicable both day and night times under all-weather conditions, which is particularly important for remote sensing since under cloudy conditions classic optical techniques are not applicable. For the Amazon basin, EDVI captures vegetation variation from dense vegetation (rain-forest) to short and/or sparse vegetation (savanna) under all-weather conditions. Good relations between microwave based EDVI and optical indexes of NDVI and EVI are found for various vegetation conditions. More importantly, EDVI shows no sign of saturation even for the tropical rain forest, while NDVI (and EVI to a lesser extent) is clearly saturated. Over the Amazon region in a normal dry season day, EDVI can provide the vegetation information over 98% of the land surface while the optical vegetation indexes can be retrieved only for a small fraction (14%) of the region.

Interannual variation in rainfall, drought stress and seedling mortality may mediate monodominance in tropical flooded forests

Flood tolerance is commonly regarded as the main factor explaining low diversity and monodominance in tropical swamps. In this study we examined seedling mortality in relation to seasonality, i.e., flooding versus drought, of the dominant tree species (Prioria copaifera), and three associated species (Pterocarpus officinalis, Carapa guianensis and Pentaclethra macroloba), in seasonally flooded forests (SFF) in Darien, Panama. Seedling mortality differed among species, years and seasons. Prioria seedlings experienced the lowest overall mortality, and after 3 years many more Prioria seedlings remained alive than those of any of the associated species. In general, within species, larger seedlings had greater survival. Seed size, which can vary by close to 2 orders of magnitude in Prioria, had a confounding effect with that of topography. Large-seeded Prioria seedlings experienced 1.5 times greater mortality than small-seeded seedlings, as large-seeded Prioria seedlings were more likely to be located in depressions. This finding suggests that seed size, plant size and topography are important in understanding SFF regeneration. For all species, seedling mortality was consistently greater during the dry season than during flooding. For Prioria, dry season seedling mortality was correlated with drought stress, that is, high mortality during the long El Niño dry season of 1998 and the normal dry season of 2000, but very low dry season mortality during the mild dry season of 1999. Prioria's ability to dominate in seasonally flooded forest of Central America is partly explained by its low drought-related mortality in comparison to associated species.

Changes in seasonal evapotranspiration, soil water content, and crop coefficients in sugarcane, cassava, and maize fields in Northeast Thailand

This study was performed to examine seasonal changes in evapotranspiration (ET), soil water content, and crop coefficients (Kc) for sugarcane, cassava, and maize fields in Northeast Thailand. ET rates during the rainy season varied between 2 and 6 mm per day but remained around 1 mm per day in the dry season. The normal dry season ET was much greater than the water loss from the top 0.5 m of soil, suggesting that capillary rise from deeper soil layers provides significant amounts of water to the upper soil layer. The Kc for sugarcane and cassava reached growing season peaks of approximately 1.10 and 1.20, respectively, in June. The maximum Kc for the maize field was approximately 1.20. Although the ET estimated by the Hargreaves equation exceeded the FAO reference ET value for this region, the values had a high correlation when the Hargreaves ET was calculated using solar radiation measurements.

Changes in seasonal evapotranspiration, soil water content, and crop coefficients in sugarcane, cassava, and maize fields in Northeast Thailand

This study was performed to examine seasonal changes in evapotranspiration (ET), soil water content, and crop coefficients ( K c) for sugarcane, cassava, and maize fields in Northeast Thailand. ET rates during the rainy season varied between 2 and 6 mm per day but remained around 1 mm per day in the dry season. The normal dry season ET was much greater than the water loss from the top 0.5 m of soil, suggesting that capillary rise from deeper soil layers provides significant amounts of water to the upper soil layer. The K c for sugarcane and cassava reached growing season peaks of approximately 1.10 and 1.20, respectively, in June. The maximum K c for the maize field was approximately 1.20. Although the ET estimated by the Hargreaves equation exceeded the FAO reference ET value for this region, the values had a high correlation when the Hargreaves ET was calculated using solar radiation measurements.

Demonstrating the destruction of the habitat of the Cape Sable seaside sparrow (Ammodramus maritimus mirabilis)

AbstractThe Cape Sable seaside sparrow (Ammodramus maritimus mirabilis) is Federally protected under the Endangered Species Act of the United States of America. This legislation prohibits direct or indirect take ‐ the killing or harming ‐ of the protected species. In 1993 and 1995, the opening of floodgates into Everglades National Park during the normal dry season resulted in a direct take of the sparrow. The argument was also made that there was indirect take through destruction of the habitat upon which the sparrow depends. Using a combination of fieldwork and satellite image analysis, we show that the floods did damage to the habitat of the sparrow. Moreover, they did so for a period longer than the actual flooding, further increasing the sparrow's extinction risk. Recovery of the sparrow population to pre‐flood levels will require an adequate and stable amount of habitat. We now have a technique for monitoring that habitat and ensuring that poor water management does not threaten it. More broadly, this technique has the potential for monitoring the habitat of many other species and avoiding another situation such as the sparrow faces.

Experimental assessment of Heliconia acuminata growth in a fragmented Amazonian landscape

Summary Fragmentation severely alters physical conditions in forest understories, but few studies have connected these changes to demographic impacts on forest species using detailed experimental examination at the individual and population levels. Using a 32‐month, reciprocal‐transplant experiment, we show that individuals of the Amazonian understory herb Heliconia acuminata transplanted into forest fragments lost over 20% of their vegetative shoots, while those transplanted to continuous forest showed a slight gain. The leaf area of plants in fragments also increased at half the rate it did in continuous forest sites. It appears that the normal dry season stresses to which forest understorey plants are exposed are greatly exacerbated in fragments, causing plants to shed shoots and leaves. The observed shifts in size could help explain why populations in fragments are more skewed towards smaller demographic stage classes than those in continuous forest. These shifts in size structure could also result in reduced abundances of flowering plants, as reproduction in H. acuminata is positively correlated with shoot number. Fragmentation‐related changes in growth rates resulting from abiotic stress may have significant demographic consequences.

NDVI anomaly patterns over Africa during the 1997/98 ENSO warm event

Normalized Difference Vegetation Index (NDVI) departure patterns for Africa during the 1997/98 El Nino/Southern Oscillation (ENSO) warm event show two dominant patterns. Over equatorial Eastern Africa, above normal NDVI anomalies persisted from October 1997 through the normal dry season (December February) and into the long rains season in March-May. Over Southern Africa the spatial NDVI anomaly shows a dry western half and a relatively greener than normal eastern half. Correlations between the temporal NDVI anomalies with ENSO indices shows that the anomalous conditions over Eastern Africa were a direct result of anomalous warming of sea surface temperatures (∼ +3 C) in the western equatorial Indian Ocean (WIO) and a lagged response to the warming in the eastern Pacific Ocean (+4 C). We suggest that this anomalous warming of the WIO and the equatorial eastern Atlantic Ocean basin dampened the normal severe drought response pattern over Southern Africa where mild drought conditions were experienced. The overall continental response pattern shows a meridional dipole pattern, with above normal NDVI straddling the equator between 10 N and 10 S and normal to slightly below normal NDVI south of 15 S, predominantly over south-western Africa.

NDVI anomaly patterns over Africa during the 1997/98 ENSO warm event

Normalized Difference Vegetation Index (NDVI) departure patterns for Africa during the 1997/98 El Niño/Southern Oscillation (ENSO) warm event show two dominant patterns. Over equatorial Eastern Africa, above normal NDVI anomalies persisted from October 1997 through the normal dry season (December-February) and into the long rains season in March-May. Over Southern Africa the spatial NDVI anomaly shows a dry western half and a relatively greener than normal eastern half. Correlations between the temporal NDVI anomalies with ENSO indices shows that the anomalous conditions over Eastern Africa were a direct result of anomalous warming of sea surface temperatures (∼+3°C) in the western equatorial Indian Ocean (WIO) and a lagged response to the warming in the eastern Pacific Ocean (+4°C). We suggest that this anomalous warming of the WIO and the equatorial eastern Atlantic Ocean basin dampened the normal severe drought response pattern over Southern Africa where mild drought conditions were experienced. The overall continental response pattern shows a meridional dipole pattern, with above normal NDVI straddling the equator between 10° N and 10° S and normal to slightly below normal NDVI south of 15° S, predominantly over south-western Africa.

Cocoa production in Ecuador in relation to dry‐season escape from pod rot caused by Crinipellis perniciosa and Moniliophthora roreri

Cropping and disease patterns were observed over a period of 4 years in individual cocoa trees at three contrasting sites near Quevedo, Ecuador, with the aim of evaluating the importance of dry season production and the consequent avoidance of pod disease caused by Crinipellis perniciosa and Moniliophthora roreri. Distinction of the diseases on necrotic pods was aided by splitting open pods after removal from the tree. The year was divided into wet season (April to September) and dry season (October to March) harvest periods. The 3‐month lag between the actual wet and dry seasons and these harvest periods allowed time for symptom development. Combined disease losses ranged from 19·0% to 62·1 % at the various sites. At all sites and in each year, more ripe healthy pods were harvested in the dry season harvest period than in the wet season harvest period. In general, pod losses caused by C, perniciosa were curtailed more sharply by the dry season than those caused by M. roreri, which at one site caused as much damage in the dry season as in the wet season. The proportion of the annual production falling in the dry season harvest period varied among sites and between years at a given site. A comparison of trees from two progenies growing side by side showed consistent differences in cropping patterns, and identified certain trees that were productive, yielded consistently in the dry season harvest period, and were little affected by M, roreri. Analysis of long‐term rainfall data for the Quevedo region indicated that years which lack a normal dry season do occur, but only once every 10 years on average. Further exploitation of disease escape through selection and breeding appears to be both feasible and appropriate.

The effects of the rainy season and irrigation on soil water and oxygen in a seasonal forest in Panama

ABSTRACTThe effects of irrigation and the rainy season on soil gases, water and physical properties were studied in a lowland moist forest in Panama. Two control plots experienced a normal four-month dry season and two experimental plots were irrigated during the dry season.The forest soils were well aerated. The average soil oxygen content at 20 cm never dropped below 15%. Irrigation in the dry season resulted in treatment effects on soil oxygen and water that persisted throughout the rainy season. The late rainy season soil oxygen content at 20 cm was 18.2% in the control plots and 16.8% in the irrigated plots. The late rainy season soil water content at 0–10 cm was 0.34 m3 m-3 in the control plots and 0.36–0.37 m3 m-3 in the irrigated plots. In the control plots, in the absence of any experimental manipulation, the soil physical properties changed within a single rainy season. Specifically, soil oxygen dropped and soil water increased during the eight-month rainy season.The observed alterations in soil physical properties probably resulted from changes in soil structure and the abundance of large pores. The rainy season or irrigation may decrease soil porosity by dispersing aggregates of soil particles, whereas soil desiccation during the dry season may enhance the formation of aggregates and large pores. As a result, the irrigated soils held more water and were less permeable to gas and water than soils experiencing a normal dry season. We conclude that the dry season may play an important role in maintaining soil structure.

A note on the modulation of Southern Oscillation‐Southern African rainfall associations with the quasi‐biennial oscillation

Since the 1950's, although the association between the Southern Oscillation and South African rainfall variability appears to have been reasonably stable, there is evidence that in certain years the association has broken down completely. The Southern Oscillation‐South African rainfall association is modulated with the phase of the Quasi‐Biennial Oscillation in tropical stratospheric zonal winds such that high phase (low phase) wet (dry) conditions occur over the summer rainfall region in January‐March only when the Quasi‐Biennial Oscillation is in the westerly phase. In contrast, there is no noticeable difference in the Southern Oscillation‐South African rainfall association between easterly and westerly years in October‐December. The Quasi‐Biennial Oscillation modulation appears to occur only when the prevalent synoptic systems over the country are of tropical origin. In the dry winter season (July‐September) the Southern Oscillation‐rainfall association and the Quasi‐Biennial Oscillation modulation are inverse to the relationships in the peak rainfall season of January‐March. Thus, a drier (wetter) than normal dry season occurs after the Southern Oscillation has been in high (low) phase and only when the Quasi‐Biennial Oscillation is in easterly, and not westerly, phase.