Water Balance Characteristics of the Salix Shelterbelt in the Kubuqi Desert
To address this gap, this study conducted systematic and quantitative monitoring within the Kubuqi Desert’s Ordos afforestation zone, Inner Mongolia Autonomous Region, China. This study delved deeply into hydrological processes and soil moisture variations during the growing season of Salix shelterbelts. Through the continuous monitoring of water–vegetation–soil, canopy interception, Salix forest transpiration, forest evaporation, soil water storage, etc., the forest water balance status was estimated. Through comprehensive evaluation, this study aimed to elucidate the characteristics of the water balance of these shelterbelts and then provide guidance on water management for ecological restoration in arid and semiarid transitional areas from the perspective of the water balance. This research poses two key questions: first, can desert willow stands in the Kubuqi Desert maintain a water balance during the growing season, and second, based on the characteristics of the water balance, can we determine how improved stand management strategies can be developed in the future?
2. Materials and Methods
2.1. Study Area
2.2. Plot Setting and Investigation
The experiment was carried out in June 2022. On the basis of the investigation of the site conditions and forest stand structure of the sample plot, a middle-aged Salix protective forest at the afforestation site was selected as the research object, and a 30 m × 30 m area was randomly selected from each forest stand. In the test plot, basic information such as plant height, diameter at breast height, and crown width was collected.
2.3. Determination of Meteorological Factors
A small automatic weather station was installed in an open and unobstructed location around the sample plot to simultaneously monitor meteorological factors outside the forest, including temperature, wetness index, wind speed, wind direction, solar radiation, and precipitation; data were recorded every 10 min. Simultaneously, three plastic rain tubes with a diameter of 20 cm were randomly placed in the open space of the sample plot for backup and correction.
2.4. Measurement of Throughfall and Stemflow in the Forest and Calculation of Canopy Interception
where TF is the throughfall (mm); VT is the volume of penetrating rain in the i-th plastic bucket (mL); m is the number of plastic barrels under the canopy; FA is the cross-sectional area of the plastic bucket mouth (cm2).
Since Salix plants are characterized by multiple branches, we applied the standard branch method to compute the total shrub-generated stemflow volume. A total of 9 Salix clumps with penetrating rain barrels were selected as objects. All base diameter dimensions of each Salix clump were measured and marked with a Vernier caliper. The average base diameter of each Salix clump was calculated; 5 branches were selected from the east, west, south, north, and middle that were closest to the average base diameter (the error in selecting branches did not exceed 3 mm); and a total of 45 stemflow collection devices were installed and arranged. The stemflow collector was affixed using hot melt glue and sealed with waterproof hot melt glue, ensuring that rainwater can flow into the plastic bottle of the collection device through the rubber tube. After each rainfall event, we utilized a measuring cylinder to quantify the stemflow volume (mL) within the bottle.
where VS is the stemflow volume of the Salix (mL); VS is the stemflow measured after rain on a single branch of Salix (mL); N is the number of Salix branches; SF is the stemflowof the Salix (mm); and SP is the projected canopy area of the Salix (m2).
where IL is the canopy interception loss (mm), Pg is the rainfall outside the forest (mm), and N is the net rainfall (mm).
2.5. Measurement of Trunk Sap Flow and Calculation of Stand Transpiration
where At, Ai, and As are the total basal stem cross-sectional area in the sampling plot, the basal cross-sectional area of stem i, and the land surface area of the plot (m2), respectively. Js is the sap flow rate in stem i (kg·h−1), is the density of water (kg·m−3), and n is the number of gauged stems.
2.6. Understory Evapotranspiration and Community Evapotranspiration
The microlysimeters were constructed using PVC pipes. The inner cylinder had an 11 cm inner diameter and was held at a height of 20 cm, with a 16 cm diameter sleeve. An impermeable layer of pearl cotton separated the sleeve and the inner cylinder to prevent rainwater from entering the gap. The bottom of the inner cylinder was fitted with 300-mesh yarn, permitting moisture passage, and securely fastened with tape (as illustrated in the image).
where ETunder is the amount of evapotranspiration under the forest on different single days (mm·d−1); Al is the surface area of the microlysimeter (cm2); is the density of water (g·m−3); and ΔMs is the daily water storage change in the microlysimeter (g·d−1).
where ET is the evapotranspiration of the Salix community, and ET is the total evapotranspiration of the Salix community.
2.7. Determination of Soil Hydrological Characteristics and Calculation of Water Storage Capacity
The soil profile was excavated at the base of the Salix forest to a depth of 80 cm for a total of 4 layers, each spaced 20 cm apart. To minimize errors, three soil samples were randomly collected from each layer using a ring knife. These soil samples were subsequently transported to the laboratory and subjected to soil water retention rate measurements via the ring–knife soaking method.
where hi and Vi are the thickness (cm) and water volume content (cm3·cm−3) of the i-th layer of soil, respectively; ΔW is the soil moisture gain; and loss (mm), We, and Wi are the water storage capacities of the 1 m soil layer at the end and beginning of the stage, respectively (mm).
2.8. Water Balance and Yield Calculations for the Sample Plots
where P is the amount of rainfall outside the forest during a certain period (mm); ET is the forest stand evapotranspiration during a certain period (mm), including canopy interception (IL) and community evapotranspiration (ET mm); ΔW is the change in water storage in the 0~60 cm soil layer during a certain period (mm); R is the yield in a certain period (including surface runoff and soil flow, mm); and Q is the exchange of water from the study soil layer to the deep layer or lateral direction during a certain period (mm). A positive value indicates that soil moisture leaks from the study soil layer to the deep layer or laterally, and a negative value indicates that the study soil layer receives water input from the deep soil or the upper slope.
During the study period, the surface runoff and in-soil flow in the soil layer in the sample plot were very low and can be ignored; R = 0. Q is a positive value and indicates that there is water side leakage from the study soil layer to the deep layer, which is the flow of forest resources in this study.
4.1. Analysis of Forestland Precipitation Redistribution Characteristics
4.2. Analysis of Forestland Transpiration and Understory Evapotranspiration
4.3. Analysis of the Total Evapotranspiration and Components of Salix
Given differences in meteorology, forest stands, soil characteristics, etc., it becomes difficult to directly compare differences in water balance between different regions. Therefore, we selected the main influential factors of the water balance for analysis and comparison. To facilitate comparisons between different studies, we expressed each water balance component as a proportion of precipitation during the same period, such as the canopy interception rate, the tree transpiration rate, the soil water storage change rate (soil water storage change/precipitation amount), and the yield flow rate. The following is a description of the total evapotranspiration and component characteristics of the Salix shelterbelt in the Kubuqi Desert during the growing season:
4.4. Soil Water Storage Change Characteristics
4.5. Shelterbelt Water Balance and Runoff Impact
The total precipitation in the 2022 growing season from June to October reached 263.2 mm. Notably, concentrated precipitation and heavy rainfall began to occur in August, causing the soil moisture content to reach a peak and the soil water storage to increase. The Q value shows that, except for Q > 0 in August, Q was negative in the other months. This shows that, except in August, the soil layer needs additional water input, and the yield is negative at this time. In fact, when we were in the sand digging a pit at the bottom of a willow (we carried out soil excavation and determined the location of the root system in order to bury the water content sensor), we found that soil at about 1 m deep was in a slightly moist state. This may be due to the recharge of groundwater or a phenomenon caused by water storage in the previous year. The counter-supply of groundwater at this time can ensure that the forest stand can still maintain normal evapotranspiration growth in June and July when there is less rainfall. When August comes, the heavy rainfall allows soil moisture to be exchanged deeply or laterally, thereby allowing the soil moisture to replenish water during later water shortages.
Overall, the low consumption of water in the study area makes it possible for the forestland to maintain its water balance throughout the growing season (for which the cumulative Q value is positive). Notably, our study mainly used the water balance method for calculation, so there may be some uncertainty in the results. To more accurately estimate the various components of the forestland water balance, future research on forest runoff and water input should be supplemented.
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