Comment on bg-2022-218

Abstract. Drained peatlands are one of the main sources of carbon dioxide (CO2) emissions globally. Emission reduction and, more generally, ecosystem restoration can be enhanced by raising the water table using canal or drain blocks. When restoring large areas, the number of blocks becomes limited by the available resources, which raises the following question: in which exact positions should a given number of blocks be placed in order to maximize the water table rise throughout the area? There is neither a simple nor an analytic answer. The water table response is a complex phenomenon that depends on several factors, such as the topology of the canal network, site topography, peat hydraulic properties, vegetation characteristics and meteorological conditions. We developed a new method to position the canal blocks based on the combination of a hydrological model and heuristic optimization algorithms. We simulated 3 d dry downs from a water saturated initial state for different block positions using the Boussinesq equation, and the block configurations maximizing water table rise were searched for by means of genetic algorithm and simulated annealing. We applied this approach to a large drained peatland area (931 km2) in Sumatra, Indonesia. Our solution consistently outperformed traditional block locating methods, indicating that drained peatland restoration can be made more effective at the same cost by selecting the positions of the blocks using the presented scheme.

The transition zones (ecotone) between boreal forests and peatlands: Modelling water table along a transition zone between upland black spruce forest and poor forested fen in central Saskatchewan

Soil carbon stocks in tropical peatlands have declined recently from water table depth (WTD) drawdown caused by increased frequency and intensity of climate extremes like El Niño and by artificial drainage. Restoration of these carbon stocks under these climatic and anthropogenic disturbances requires improved predictive capacity for hydrological feedbacks to ecological processes. Process‐based modeling of tropical peatland ecohydrology could provide us with such capacity, but such modeling has thus far been limited. We aimed at using basic processes for water and O2 transport and their effects on ecosystem water, carbon, and nitrogen cycling to model seasonal and interannual variations of WTD and surface energy exchange. We tested these processes in a process‐based model ecosys in a drained tropical Indonesian peatland from an El Niño year 2002 to a wetter year 2005. WTD was modeled from hydraulically driven water transfers controlled vertically by precipitation versus evapotranspiration (ET) and laterally by discharge versus recharge to or from an external reference WTD. These transfers caused WTD drawdown and soil drying to be modeled during dry seasons, which reduced ET and increased Bowen ratio by lowering stomatal conductance. More pronounced dry seasons in drier years 2002–2004 versus wetter year 2005 caused deeper WTD, more intense peat drying, and greater plant water stress. These modeled trends were well corroborated by site measurements as apparent in regression statistics of modeled versus observed WTD (R2 > 0.8), latent heat (R2 > 0.8), and sensible heat (R2 > 0.7) fluxes. Insights gained from this modeling would aid in predicting the fate of tropical peatlands under future drier climates.

The relationship of δD and δ18O in soil water and its implications for soil evaporation across distinct rainfall years in winter wheat field in the North China Plain

The present study investigates the year-to-year variations of September–October rainfall in Hainan, China, for the period 1965–2010. The dominant circulation anomalies feature a cyclone (an anticyclone) over the Indochina Peninsula and northern South China Sea, an anticyclone (a cyclone) over subtropical western North Pacific and lower-level convergence (divergence) over the Maritime Continent in the wet (dry) years. These circulation anomalies are responses to an east–west sea surface temperature (SST) anomaly pattern with negative (positive) SST anomalies in the equatorial central Pacific and positive (negative) SST anomalies around the Maritime Continent in the wet (dry) years. Although the SST anomaly pattern is similar (but with opposite anomaly), the SST signal in the equatorial central Pacific is more significant in the dry years than in the wet years. This difference indicates a larger case-to-case variability in the wet years than in the dry years. The large variability in the wet years is attributed to contributions of tropical cyclones (TCs) and intraseasonal oscillations (ISOs). There are more TCs impinging on Hainan and the TC tracks are closer to the island in the wet years than in the dry years. The rainfall shows large intraseasonal variations with periods of 10–20 and 30–60 days during September–October in the wet years. The 10–20-day ISO originates from the Maritime Continent, whereas the 30–60-day ISO develops over tropical Indian Ocean and propagates northeastward to northern South China Sea. In contrast, the ISO signal is much weaker in the dry years.

Hydrological models often perform poorly in simulating dry years in regions with large inter-annual variability in rainfall. We calibrated the Soil and Water Assessment Tool (SWAT) model to dry and wet years separately, using the semi-arid Barrett watershed on the west coast of USA as an example. We used hydrological and meteorological data from 1980–2010 to calibrate the SWAT model parameters, compared the monthly runoff results simulated by SWAT using a traditional calibration for the entire runoff series with results using a calibration with the wet and dry year series, and analyzed differences in the most sensitive parameters between the wet and dry year series. The results showed that (1) the SWAT model calibrated to the entire runoff series produced significant differences in simulation efficiency between the wet years and dry years, with lower efficiency during the dry years; (2) the calibration with separate wet and dry years greatly enhanced the SWAT model’s simulation efficiency for both wet and dry years; (3) differences in hydrological conditions between wet and dry years were represented by changes in the values of the six most sensitive parameters, including baseflow recession rates, channel infiltration rates, Soil Conservation Service (SCS) curve number, soil evaporation, shallow aquifer flow, and soil water holding capacity. Future work can attempt to determine the physical processes that underlie these parameter changes and their impact on the hydrological response of the semi-arid watersheds.

Peatland fires occur almost every year in Indonesia that might be caused by over-drained peatland due to canalization for a certain purpose. Canalization in the peatland leads to a significant decreasing change of groundwater level that might cause peatland very vulnerable against fire. The rewetting approach by the Peatland Restorations Agency of Indonesia using canal blocking is expected to keep groundwater level high so that the peatland is maintained in an always wet condition. Therefore, it is important to study the impact of canal blocking on the fluctuation of groundwater level in peatland ecosystems. This study was carried out at a research site which is located in Pulau Tebing Tinggi peatland hydrological unit, Kepulauan Meranti Regency, Riau, Indonesia. Groundwater level fluctuation as the impact of canal blocking was monitored using eight dipwells that were set with a distance of 1 m, 51 m, 101 m, and 201 m from the canal for each transect of the three transects. The result of this study showed that the canal blocking has a good impact for keeping groundwater level and keeping peatland on an always wet condition for peatland restoration and peat fire prevention until 400 m distance to the upstream area from canal block and about 1 m distance perpendicular to the canal. The canal blockings will have a good impact on rewetting at the peatland area up to 201 m distance perpendicular to the canal if the rise of water level at the canal is more than 0.6 m for peatland restoration.

The effects of two water table depths (WTD), 0.6 and 0.9 m below the soil surface on subsidence, subsidence rate and swelling of an organic soil were observed in large undisturbed cores under greenhouse conditions. Measurements were made in two tiers, 0–0.3 m (top tier) and 0.3 m to WTD (bottom tier) during the growth of a carrot crop with WTD as above, and continued following a rise in the water table. The WTD of 0.9 m caused the top tier to subside twice as much as the same tier in the 0.6 m WTD. Top tier subsidence seemed irreversible in both WTD because minor swelling was observed following a rise in the water table. Most of the reversible subsidence occurred in the bottom tier. Raising the water table reduced the total profile subsidence by 36 and 24% for the 0.6 and 0.9 m WTD, respectively. After correction for oxidation, subsidence accounted for 3.2 and 5.9% of the 0.6- and 0.9-m profiles, respectively. The bulk density increase in the 0.6-m profile before and after the end of the experiment was not significant but a significant increase of 11% was measured in the 0.9-m profile. Subsidence rates decreased in both tiers during the growing period reaching a minimum in both WTD at harvest time. The subsidence rate in 0.9-m profile at 100 days after seeding was 2.5 times the rate in the 0.6-m profile. Key words: Organic soil, subsidence, swelling, water table

Due to the temperate and relative dry climate of south-eastern Australia, there are relatively few moisture-sensitive palaeoenvironmental records in the region. The region is of significance as it is influenced by several major ocean-atmospheric modes including El Niño–Southern Oscillation (ENSO) and the Inter-decadal Pacific Oscillation (IPO). In this research we applied a transfer function between (sub)fossil testate amoebae and water table depth to consider local surface wetness, which may reflect effective moisture conditions, at Snowy Flat, a high-altitude Sphagnum -shrub bog in south-eastern Australia over the late Holocene. The reconstruction water table depth, covering the last ∼3100 cal yr B.P., suggested several distinct periods of enhanced surface wetness, perhaps reflecting moisture availability, including from 2400 to 2200, 1350–1100, 920–860 and 670 to 60 cal yr B.P.. After 1800 cal yr B.P., depth to water table (DWT) was more variable and followed well-known trends of the Northern Hemisphere including the Medieval Climatic Anomaly (MCA; increased DWT) and the Little Ice Age (LIA; decreased DWT). We also investigated the instrumental record of rainfall and found a significant negative relationship with the IPO and an IPO-like tripole spatial pattern between rainfall and sea surface temperatures in the Pacific Ocean. This suggests that the late Holocene hydroclimate at the Snowy Flat site, and across south-eastern Australia and more widely, can be related to the IPO and this is especially true after 1800 cal yr B.P. when the relationship apparently strengthened. The reconstruction also indicated that a positive IPO (El Niño-like) mean state prevailed in the MCA and a negative IPO (La Niña-like) mean state dominated the LIA. Surface moisture (DWT) was also related to total solar irradiance (from 10 Be in polar ice): periods of enhanced surface moisture (reduced DWT) (mean negative IPO states) coincided with minima (e.g. 920–860 and 670–60 ca. yr B.P.) and reduced moisture (increased DWT) coincided with maxima (e.g. 850–700 cal yr B.P.). Together these results shed further light on the variability of moisture in south-eastern Australia, and further afield, and the potential mechanisms for these. • Water table depth was reconstructed, over the last ∼3.1 ka, using testate amoeba. • Wetter periods were found 2500–2200, 1350–1100, 920–860 and 670–60 cal yr B.P. • The MCA/LIA were periods of reduced/enhanced moisture availability. • Enhanced moisture availability reflected negative IPO and lower solar irradiance.

In the southeastern United States, coastal communities face challenges for water resources and wastewater treatment capacity. In North Carolina, 51 municipalities irrigate forests with municipal wastewater to absorb nutrients, reduce direct effluent discharge to surface waters, and recharge groundwater. Most facilities have land‐applied wastewater for decades, but there are no quantitative studies on the hydrologic impacts of this practice. This study developed a simulated water balance for the largest forest land‐application system in North Carolina which treats wastewater daily by irrigating 30 km2 of a mixed hardwood‐loblolly pine forest. A distributed hydrological model (MIKE SHE) was adapted to simulate 20 years of watershed evapotranspiration (ET) and water table depth (WTD) under irrigated and nonirrigated conditions. We found that irrigation impact to annual and monthly WTD was negligible in years with average and above average rainfall. For wet years, drainage increased with irrigation while ET and WTD remained similar to nonirrigated conditions. In dry years, ET was 31 to 39 mm higher in irrigated forest than nonirrigated forest though the change in groundwater storage remained close to zero annually. Our simulation study suggested that the drivers of on‐site drainage were predominantly rainfall and irrigation, and the annual watershed drainage increased in volumes equal to 93%–100% of the added annual irrigation input. This study offers insights to water balance dynamics in irrigated forests and coastal forest resiliency to variable wastewater hydraulic loading.

This study utilizes a linear, primitive equation spherical model to study the development and propagation of easterly wave disturbances over West Africa. Perturbations are started from an initial disturbance consisting of a barotropic vortex and the governing equations are integrated forward in time. The perturbations are introduced into basic states corresponding to the observed dynamical and thermodynamical characteristics of two wet years in the Sahel and two dry years. The model simulations show consistent contrasts in wave activity between the wet and dry years. The waves are markedly stronger in the wet years and show a barotropic structure throughout the troposphere. The waves tend to extend throughout the troposphere to the level of the tropical easterly jet (TEJ) in the wet years, but not in the dry years. The upper-tropospheric shear, which is stronger in wet years, appears to be a key factor in wave development. This shear is dependent on the intensity of the TEJ, suggesting that the TEJ is an important factor in interannual variability in the Sahel. When the overall shear is weak, vertical development is suppressed. Another contrast is that in the dry years the growth rates show a single maximum around 3000–4000 km, but in the wet years there is a second, around 6000–7000 km. This suggests that both synoptic-scale and planetary-scale waves are active in the rainy season of some wet years. Imposing considerations of potential vorticity, the generation of planetary-scale waves implies a strong link between the surface and the TEJ in wet years. Such a link is absent in the dry years. This is likely a major factor in the interannual variability of rainfall in the Sahel.

Interannual rainfall extremes over southwest Western Australia (SWWA) are examined using observations, reanalysis data, and a long-term natural integration of the global coupled climate system. The authors reveal a characteristic dipole pattern of Indian Ocean sea surface temperature (SST) anomalies during extreme rainfall years, remarkably consistent between the reanalysis fields and the coupled climate model but different from most previous definitions of SST dipoles in the region. In particular, the dipole exhibits peak amplitudes in the eastern Indian Ocean adjacent to the west coast of Australia. During dry years, anomalously cool waters appear in the tropical/subtropical eastern Indian Ocean, adjacent to a region of unusually warm water in the subtropics off SWWA. This dipole of anomalous SST seesaws in sign between dry and wet years and appears to occur in phase with a large-scale reorganization of winds over the tropical/subtropical Indian Ocean. The wind field alters SST via anomalous Ekman transport in the tropical Indian Ocean and via anomalous air–sea heat fluxes in the subtropics. The winds also change the large-scale advection of moisture onto the SWWA coast. At the basin scale, the anomalous wind field can be interpreted as an acceleration (deceleration) of the Indian Ocean climatological mean anticyclone during dry (wet) years. In addition, dry (wet) years see a strengthening (weakening) and coinciding southward (northward) shift of the subpolar westerlies, which results in a similar southward (northward) shift of the rain-bearing fronts associated with the subpolar front. A link is also noted between extreme rainfall years and the Indian Ocean Dipole (IOD). Namely, in some years the IOD acts to reinforce the eastern tropical pole of SST described above, and to strengthen wind anomalies along the northern flank of the Indian Ocean anticyclone. In this manner, both tropical and extratropical processes in the Indian Ocean generate SST and wind anomalies off SWWA, which lead to moisture transport and rainfall extremes in the region. An analysis of the seasonal evolution of the climate extremes reveals a progressive amplification of anomalies in SST and atmospheric circulation toward a wintertime maximum, coinciding with the season of highest SWWA rainfall. The anomalies in SST can appear as early as the summertime months, however, which may have important implications for predictability of SWWA rainfall extremes.

Carbon dioxide (CO2) efflux from soil (or soil respiration, SR) is one of the most important yet variable characteristics of soil. When evaluating large areas, CO2 efflux modeling serves as a viable alternative to direct measurements. This research aims to identify site-specific differences and their effects on empirical CO2 efflux modeling. The experimental data from 25 years of field observations were utilized to identify the optimal site- and weather-specific models, parameterized for normal, wet, and dry years, for the forest and grassland ecosystems located on similar Entic Podzols (Arenic) in the same bioclimatic coniferous–deciduous forest zone. The following parameters were considered in the examined models: mean monthly soil or air temperatures (Tsoil and Tair), amount of precipitation during the current (P) and the previous (PP) months, and the storage of soil organic carbon (SOC) in the top 20 cm of soil. The weighted non-linear regression method was employed to estimate the model parameters for the normal, wet, and dry years. To increase the magnitude of the model resolutions, we controlled the slope and intercept of the linear model comparison between the measured and modeled data through the change in R0—CO2 efflux at Tsoil = 0 °C. The mean bias error (MBE), root-mean-square error (RMSE), and determination coefficient (R2) were employed to assess the quality of the model’s performance. The measured Tsoil, Tair, and P, as well as the litter (for forest) or sod (for grassland) horizon (modeled by the Soil SCLmate Statistical Simulator (SCLISS)), and soil temperatures (Tlit_m, Tsoil_m) and moistures (Mlit_m, Msoil_m), were used for SR simulation. For the CO2 efflux in the forest ecosystem with the lower SOC availability for mineralization, the direct Tsoil and Tair measurements in combination with SOC storage provided better parameterization for the empirical TPPC model. For the CO2 efflux in the grassland ecosystem with the high SOC availability for mineralization, the temperature became the governing factor, and the TPPrh model provided better performance over all the considered models. The model’s performance was the best for the wet years, and the worst for the dry years for both ecosystems. For forest ecosystems, the model performance for average precipitation years was equivalent to that in wet years. For grassland ecosystems, however, the model performance was equivalent to that in dry years due to differing exposure and hydrothermal regimes. The wet-year R0 obtained for both forest and grassland ecosystems differed from the normal- and dry-year values. The measured SR values relevant for the R0 estimations distribute along the precipitation range for the forest and along the temperature range for the grassland. The SCLISS-modeled Tlit_m and Mlit_m provide good alternatives to direct atmospheric measurements, and can be used as initial temperature and moisture data for CO2 efflux modeling when direct soil and moisture observations are not available on site.

Controlled drainage (CRD) is an agricultural water management practice designed to adjust the capacity of a drainage system under varying hydrological conditions. This simulation study aimed to quantify the potential of combining a controlled subsurface drainage (CS) with open ditch damming (CD) to manage the water table depth (WTD) and field water balance in Nordic conditions. Simulations with and without controlled drainage were run using a hydrological model that had been set up for a flat loamy field in Northern Ostrobothnia, Finland, for the period 2010–2021. All CRD scenarios reduced the probability of deep WTDs during growing seasons (May–Sep). The impact of CS on WTDs was greater and more uniform than CD. The CRD effects on water balance were seen in water outflow pathways, as CS reduced drain discharge while CD had the opposite effect. When both methods were applied simultaneously, annual evapotranspiration increased 5–12% compared with the free drainage scenario. The effects of CRD on evapotranspiration were greatest during the dry years indicating that CRD has potential to reduce drought in food production areas. None of the CRD scenarios could maintain optimal WTDs during the entire growing season, highlighting the complexity of optimizing field water management using CRD alone.

Saguling Reservoir has a potential to be used as a raw water supply for Bandung Metropolitan Area (BMA), with the discharge of 1.622 dm·s−1. However, further studies are needed to ensure that the water quality is in accordance with the government regulations. This study shows that the reservoir’s P concentration was 315.0 mg·m−3 on average in 1999–2013. This value only meets the class III of government standards of water quality for the cultivation of freshwater fish, livestock, and to irrigate landscaping, but does not belong to the class I standard of 200 mg·m−3 for drinking water. The total-P concentration in wet, normal, and dry years was 796.3, 643.8, and 674.8 mg·m−3, respectively. The pollution load was highest in wet years due to the high levels of sediment. The pollution load of the reservoir did not exceed the class III classification of 29 405.01 kg·year−1. The pollution load in wet, normal, and dry years was 38 790.1, 25 991.9, and 23 929.0 kg·year−1, respectively. The phosphorus pollution is caused by the use of floating net cages in the reservoir, which makes it difficult to meet the standards. In wet years, the pollution load was higher than in normal and dry years. The P load could be higher in the wet season due to dilution and could probably decrease the pollutant concentration in the reservoir.