Agro-meteorological Aspects in Water shed Management
Rain fall is the driving force in water shed programme. Rainfall variability in India in terms of its spatial and temporal dimensions is the rule rather than the exemption and hence water shed management gains important to conserve and utilize both green and blue water to produce more crops per drop of water. Green water represents the moisture contained in the soil profile from rain/ irrigation, while the blue water accounts for the run-off from irrigation /rain fall and contained in water bodies such as dams, rivers, lake and aquifers. Green water scarcity as a result of erratic rainfall pattern can be compensated by blue water, if stored run-off water is available. Watershed programme has optimized the conservation and use of green water, while run-off water must be used cautiously through suitable crop rotation. In the event of anticipated climate change, the only available comprehensive technology is the scientific management of water shed. At this context some agro-meteorological tools, which are most relevant to scientific management of watershed, are discussed here under.
1. Water balance or water budgeting
- Developing a strong water management strategy for any purpose necessarily requires information on water balance of a particular domain.
- The wetness or dryness of a place is determined by the relative magnitude of precipitation and PET, as described by Thornthwaite.
- In applying the water balance to a particular problem, the boundaries to the system must be delineated and inflow, out flow and storage terms appropriate to the system must be carefully defined.
In literature general-purpose water balance models and crop specific models are available but in this schedule, only a few models have been described.
Water balance can be defined as a method to account the receipt and expenditure of soil moisture in a given situation.A simple soil water balance can be written as follows (Venketa Raman and Krishnan, 1992).
P = E + RO + UD +DW
Where P, RO and UD are precipitation, run off and deep drainage respectively, E is evapo transpiration and DW is the change in soil water storage (W) during the period.
During rain less situation P, RO and UD values are generally negligible and hence the equation E =D W may be considered for water balance studies.
Thornthwaite Book keeping water balance model (Thornthwaite, 1948; Thornthwaite and Mather, 1955).
This is a standard method and is widely used. Mean monthly rainfall (Minimum average of thirty years), monthly PET (Based on penman method) and soil field capacity in mm (based on analysis) are the input data.
By using book keeping procedure it is possible to arrive accumulated potential water loss, storage change, actual evaporation, water deficit and water surplus. A skeleton of the table is given for understanding.
Field capacity: -------mm.
P-PE Values = +ive values P-PE value =-ive values
Net value = + ---------
Keig and Alpine water balance:
It is a simple and non crop specific water balance model developed by keig and Mc Alpine (1974).This model requires the inputs such as weekly rainfall, weekly evapotranspiration and soil moisture at the beginning of the week. The initial soil moisture at the beginning of the entire analysis is taken as one-fourth of the water holding capacity that was specified. The program then generates a moisture response factor (A) to take into account the effect of soil moisture depletion. The model assumes that for the soil moisture depletion range of 1 to 38%, the response factor (A) is 1.00. In the other ranges of soil moisture depletion, the response factor is assumed to be varying linearly. The response factor will be 1.0 at 38% depletion and 0.01 at 100% depletion of soil moisture. Thus, the values of A for different levels of soil moisture depletion are generated in the beginning of the analysis by the model and stored for further use when the yearly and weekly loops are encountered.
In the weekly loop, the weekly rainfall values vary from year to year whereas the weekly ET values are the same over the years since only long-term ET values are considered to represent the water demands. The WATBAL program estimates the percentage of available moisture in relation to the water holding capacity (MIN). The initial soil moisture is added to the rainfall to obtain the total stored moisture, which is then divided by WHC and expressed as MIN in percentage. Depending upon this percentage, the appropriate response factor (A) from the A matrix, that has been already created, is taken to obtain the factor AE/PE. In other words, AE/PE is expressed as A (101-MIN). The argument represents the percentage as an integer and the corresponding response factor from A matrix is taken as the AE/PE ratio. For example, let the response factor for a soil moisture depletion level of 26% be 0.7, say. If the initial soil moisture 25mm and rainfall during the week is 50 mm, then.
MIN = { (25+50mm) / 100mm} * 100 = 75%
The AE/PE ratio is A (101 – 75) = A(26) = 0.7
After having determined the AE/PE value, the WATBAL program then computes actual evapo transpiration (AE), which is the product of AE/PE and PE. If the current soil moisture is zero, then there will be no runoff in the week and the terminal soil moisture will be zero. Even if the current moisture is significantly non-zero, if AE is greater than the current soil moisture, then also the runoff and terminal soil moisture terms will have zero values. On the other hand, if AE is less than the current soil moisture, it is assumed that the available moisture is not fully exhausted, leaving the balance as the terminal soil moisture. If the terminal soil moisture is less than WHC, then runoff is not taking place. Contrarily, the increase in terminal soil moisture over WHC will lead to a runoff event. In this case, the quantity of runoff will be equal to the excess soil moisture over WHC. Here, the terminal soil moisture is consequently limited to WHC.
The model format is as follows.
WATBAL
AWHC = mm
2. Length of Growing Period:
This is also called as Effective Growing Period (EGP)..Different methods are available viz, Weekly Moisture Index method (Krishnan and Rao, 1979), Higgins and Kassam,(1981) FAO model, Weekly R/PE method of J.Reddy(1983) Weekly Moisture Availability Index method of Sarkar and Biswas(1988) and Sastry method and among them the FAO model is simple and relevant.
Assessing Effective Growing Period (EGP) on practical basis is very important, especially for dry lands, since this period provides the required optimum soil moisture to the growing crops.
In this FAO water balance model mean monthly rainfall and 50 per cent PET values were plotted for twelve months. Whenever, the monthly rainfall did cross the 50 per cent value of PET in a concerned month, that month was taken as starting period of the EGP. Similarly, when the mean monthly rainfall got down from the monthly 50 per cent PET value, that month was taken as termination of the EGP. In addition, the stored soil moisture after the termination of the EGP was also taken into account to compute total EGP.
3. Rain fall analyses:
Rainfall analyses are very important for farm planning especially under watershed programme and also for taking weather based decisions. The analysis types are given here under;
Moisture level at the beginning of the model----------mm
The procedure to use these analyses can be referred in he book on Crop Planning-Climate Atlas; principles (Veeraputhiran et al, 2003).For a water shed area of 500 ha a minimum of 25 rain gauges must be installed around the water shed and the collected data from these rain gauges must be put under weighted average and used for further analyses.
4. Integrated Weather forecast and Weather Based technologies
Integration means bringing together different components of a system for effective synergistic operation and impact. This is mostly required for Indian weather forecast system being given by different organizations under different spatial and temporal modes.
Four activities must be carried out for efficient weather forecast system and they are;
- Single window must be identified to disseminate now cast, short range, medium range and seasonal climate information to the farmers. This may be done either by IMD or NCMRWF or concerned State Agricultural University,
- For carrying out this exercise all the three organizations must work together with close understanding and co-operation,
- Educating farmers on weather consciousness and using valuable information effectively, and
4. Identification of selected mass media for communication.
For each agro climatic zone a separate agency may be identified and all forecast information must be given to the farmers through this single window as indicated here under. The existing window in India is NCMRWF, which has strong communication channel and this mode may be used for this purpose.
Through this window, first the weather information from long range and seasonal climate forecast must be given to the farmers. This will help the farmers to take farm decision on crops to be selected, area to be brought under crops during the season and best bet technology to be selected and it’s tailoring. Further, this will help the farmer to procure required input for the ensuing season and also to facilitate farmer to arrange for loan. Comprehensively the National Government can decide the policy of export and import based on the seasonal forecast cropping programe.
Secondly, after giving long range forecast information, the information from medium range weather forecast must be given to farmers through the same window continuously to take farm decision on preparing the field for sowing, organizing laborers, pest and disease management, preparing contingency plan to meet both benevolent and malevolent weather situations and farm operations which require 3 to 4 days advance time for pro action planning.
Thirdly, the information from short range may be given to take farm decision on sowing, irrigation scheduling, weeding, fertilizer application, plant protection, harvesting etc,
Fourthly the farmers may use the now casting information for mid term correction of their farm operations proposed.
In this way all the forecast information must reach through single window to the farmers one by one with in an agricultural season.
Benefits of this integrated weather forecast system
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