Deep soil drying due to artificial plantations is an important ecological issue in semiarid regions where ground water is generally unavailable. I have daily soil moisture data at deep layers (θd) in a semiarid plantation on the Loess Plateau. And I also have daily precipitation (P), daily transpiration (T) and daily shallow soil moisture (θs) as affecting factors. My purpose is to quantify the contribution of P, T and θs to the changes of θd, but I am not sure which method should be used. Thanks a lot for any help in advance.
I don't know if my personal approach tomsuch data would be helpful. If you have your data in Excel or similar, I would plot the time series data by day on same plot, and as needed, extend the x axis (time) to a width, so the daily detail can be viewed. If preferred, daily transpiration can be plotted as negative values, to separate from the positive daily rainfall. Upon reviewing, you may find that the daily daa is either not enough detail, or too much detail. You may want to compare growing season responses in soil moisture changes when transpiration is low in dormant season. I note you did not mention runoff data also, so if it occurs, the water balance approach will show areas difficult to model, especially for severe events with stormwater discharges. There are various ways to conduct water balance, addressing water inputs and outputs. With your data, if you select periods that begin and end at a common dry soil moisture level, you can discuss the inputs P and outputs T (and runoff if known). Within these moisture wetting and drying cycles or periods, the soil moisture when plotted daily reveals something similar to a stream hydrograph, so there will be one or more peaks, then periods of no rain when there is a recession curve, which will be mor abrupt in growing season than dormant. Unsure about detail and depth of soil moisture readings, if you do not have water table wells for groundwater, there might be some deep moisture recharge or losses if groundwater is very deep or not present that would not be addressed. In addition, if groundwater recharge comes from outside sources such as uplands and reappears in lowlands, you would want to know if it affects your study areas, such as no rainfall inputs and soil moisture is increasing. From my experience, by plotting the data that you have and inspecting it in detail, running some water balancing calculations, looking at soil recharge and drying cycle curves (or what I sometimes refer to as groundwater hydrograph), you will be in a better position to decide if this is enough for your questions, or how to proceed in adjusting to more frequent data collection, or moving to weekly, monthly, dormant and growing cycles, moisture wetting and drying cycles.
Although my semiarid experience is lacking, vegetation as well as lack ot it can have substantial effects to the hydrology of these areas. Dry areas that historically were native prairies of drought tolerant native grasses, have been overgrazed or abused to the point of becoming areas with higher runoff and erosion with sparse cover. Luna Leopold in one of his western USA papers discussed how the dry periods essentially had more erosion than wet periods, due to change in vegetation cover during these periods.
Dear Xiaodong,
Plot the time series of the soil water storage is a proper start to analyze the impacts of the affecting factors. Seasonally, the deep soil water content may affected by percolation from upper soil layers due to heavy rainfall event, while its depletion is mainly caused by the root uptake of transpiration. It will be good to plot the seasonal changes of both soil water storage and soil water profiles to identify the infiltration and root uptake.
In Loess Plateau, evolution of the dry layer in deep soil due to plant utilization seems very complicated. It is an issue of long-term interactions among precipitation - soil water - plant water use. Observation data over short term, say one or two years, seems not sufficient to reveal the interactions.
If you hope to reconginize how the infiltration and root uptake affect the soil mositure in deep layers, a SPAC model may be a useful tool.
Good luck.
Luo
It is known that acid rain, a product of atmospheric pollution, leaches the metals needed by plants, thus destroying the soil's ability to sustain life.
The same happens with the mine waters that reach the soils, since they are acid waters, in addition with heavy metals.
The only way to get an understanding of these complex systems is to use a water balance model. This is a computer program, where transpiration , percolation stored water etc.... are all calculated from rainfall and evaporation potential - usually on a daily basis. A summary of the process is at http://www.eoc.csiro.au/aciar/book/PDF/Monograph_84_Chapter_01.pdf There have been many programs written for this purpose - some are simple -designed for use by farmers https://grdc.com.au/news-and-media/news-and-media-releases/south/2016/01/new-app-monitors-soil-water
some are much more complex and include landscape features. eg https://www.pcswmm.com/?gclid=EAIaIQobChMI4bLFyYyU1QIVxBmPCh38SgD1EAAYASAAEgIDfvD_BwE
I would recommend something between these two extremes. if you have programming skills I can offer you source code for a model I used successfully for many years.
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