Xiaodong,

Yes, you procedures are reasonable. However, I would not throw out the 0-10 cm. If the plant covers the surface, (no bare ground) the soil evaporation will be small. Most of the change in moisture will be transpiration. Are you also measuring the rainfall?

Glenn

Xiaodong

Yes, I agree with Dr. Glenn. Your procedures are reasonable, but the soil moisture in the 0-10 cm can be important because that layer participate on water balance in the 20-280 cm. If you consider rainfall, positive or negative values in the layer of 0-10cm will influence on water balance in the layer of 20-280 cm.

Edson Vieira

I agree with previous recommendations. Bet way to get a good understanding of the flows is to model water balance in each soil profile (8 depth intervals as you mention). You can model δi for each as the sum of inflows from previous profiles minus the sum of outflows into the next profile. Some important components to include in each profile are net precipitation, infiltration rates, plant uptake, soil evaporation, etc. Unknown variables that are not measured could be derived from the water balance. A suggested equation in each interval is:

δi=Incomiing Infilitration+subsurface inflows-outflows to next in interval-plant uptake-Evaporation-subsurface horizontal outflow.

Hope this helps.

Without defining actual rootzone depth, all above comments may mislead

Dear Xiaodong Gao,

Please mention the crop first. This is because the plant water uptake will depend on the root zone depth (in fact the word "Effective Root Zone" is more appropriate) which is different for different crops. From your description I understand that it would be a tree or any deep rooted crop, otherwise monitoring upto 280 cm is too much.

Secondly, what is the frequency and average amount of rainfall in your study area? Is it enough to percolate as deep as 280 cm?

Third, if its an agricultural crop, ignoring upper 10 cm is a big mistake since most of the water and nutrient uptake in such crops occur in the plough layer. You must record these data too. However, you can later decided to keep or discard these data, depending upon your objectives.

Dr. Zafar

Hello, if you have water potential measurements you can use those to help determine the direction of water movement within the soil too. 

Dear All,

Thanks a lot for your helpful answers. Some more details. First, the plants in our site are trees. Second, the annual mean precipitation is 470 mm, a typical semiarid climate. And the maximum infiltration depth in a normal water year is ususally within the 200 cm. But the rooting depth can be greater than 300 cm, and thus the soil water in the 280 cm can be used by trees although the magnitude is low. Thanks again for all.

Xiaodong

Dear Xiaodong Goa,

OK, now its clear. Keeping in view the plants and rooting depth, I think your procedure is acceptable. However, I still recommend you to consider the upper 10 cm as well. Try to assess the effect of evaporation on soil moisture changes in this layer. This can be done be simultaneously observing another adjacent soil having no plant cover.

Regards

Dr. Zafar

A better way to find out the root uptake profiles is mathematical modeling. The observed soil water profiles indicate the soil water status instead of root uptake. The change in soil water content within a soil layer does not mean the amount of root uptake. Water uptaken by roots in a layers may come from some other layers due to soil water movement, or even from soil layer below 280 cm. There are a school of such models you can use. Some of them are pretty simple which are not data intensive.