Moisture Content
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Increase soil moisture in the floodplain forest area
Improving
Iori River Valley
Georgia
Problem: The Dali dam construction began on the Dali river in 1984, and since its completion in the 1990s has led to a reduction in soil moisture in this area. As a result, the floodplain forest downstream of the dam has been drying out, suffering a loss of native floodplain forest tree species. The floodplain forest extent and condition has been declining, in part due to a lack of regeneration of new trees.
Action: SABUKO is working with the dam managers to recreate flooding events by periodically releasing water from the dam. They have also constructed a gabion to retain water in the floodplain area when water is released.
Indicator: an increase in soil moisture indicates the presence and retention of water in the area, a critical step towards promoting the natural regeneration of flood-tolerant or flood-requiring tree species that will comprise the next generation of native floodplain forest trees.
Methods
Moisture content will be indicated by the Normalised Difference Water Index (NDWI), which is strongly related to plant water content. This information is derived from satellite (Sentinel 2) imagery, which allows for the spectral analysis of near-infrared and short-wave infrared pixels in the image. Resulting values range from -1, indicating completely dry area (without vegetation or with dried vegetation), to 1, indicating very moist (leaves saturated with chlorophyll and therefore, water). These values were derived from the same area in 2019, before any restoration actions occurred, and in 2021 and 2022, after the periodic flooding began in 2020.
The construction of the gabion, which would help retain water flowing out from the river in the floodplain area. Photo: SABUKO
Equipment used to measure moisture content. Photo: Nika Tsitelashvili
Measuring the dam’s water level. When the water level is sufficiently high, water is released to simulate a flooding event. Photo: SABUKO
Fieldwork assessing the water level around the gabion. Photo: Giorgi Guliashvili
Results
Compared to 2019, NDWI values in 2021 and 2022 show an increase in water content in the floodplain forest area, based on a month-by-month analysis shown below.
Interpretation
The maps depicted above show the change in moisture (represented by NDVI values) from before and after restoration interventions. A different map is shown for each month, because moisture levels fluctuate monthly, due to seasonal and rainfall patterns. The red areas show where moisture has been lost, and this was observed above the Dali dam (in the control area where no restoration has taken place). Below the dam (the restoration area) saw less loss of moisture over the same period.
On the ground we have observed a retention of water surrounding the gabion (approximately 2 ha); however those effects are too small and localised to be viewed by satellite imagery, and therefore are not captured here. Overall, this initial data shows a difference between the control and restoration areas over three years, across four key months. These differences are likely due to a combination of factors (precipitation, vegetation growth, reservoir water level fluctuations, and seasonality). Continued monitoring of the NDVI will allow us to better understand all the factors contributing to moisture retention (or loss), and show the longer-term effects of restoration treatments.
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