vinay sehgal iari, new delhi -...
TRANSCRIPT
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SOIL MOISTURE BASED INDICES
FOR DROUGHT MONITORING
Vinay Sehgal
Division of Agricultural Physics
IARI, New Delhi
THE NEED FOR A DROUGHT MANUAL
� Handbook for all the decision-makers / disaster managers,
from the National to village-level
� To reflect the new framework for drought management
(monitoring, declaration, response, and mitigation etc.) as
a continuum of activities.
� To introduce and institutionalize a new drought
management system, which is based on the technological
advances and new innovations in crop and water
management.
� Information on mitigation measures both short-term (relief
etc.) and long-term ( mitigation ) strategies for an effective
response (linkages with the existing development
programmes).
� Renewed approach to drought management
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BACKGROUND
�Drought is complex phenomena
�Slow onset
�Multiple impacts
�Agricultural impacts are the focus in
India
�Effective Drought Monitoring Systems is
the precursor to Drought Declaration
� Invest in DMC
BACKGROUND: FIVE CATEGORIES OF INDICES
FOR MONITORING/DECLARING DROUGHT
� Rainfall
� Rainfall deviation / Standardized Precipitation Index
� Dry Spells
� Crop Situation based indices
� Area under crop sowing
� Percent Available Soil Moisture (PASM)
� Moisture Adequacy Index (MAI)
� Vegetation (Remote sensing based indices)
� NDVI deviation
� NDWI deviation
� VCI (NDVI)
� VCI(NDWI)
� Water
� Surface: Reservoir storage Index
� Groundwater Drought Index
� Streamflow Drought Index
� Others (socio-economic indicators)
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AREA UNDER SOWING
� Being monitored by State Department of Agriculture
� Need to collate data at required administrative unit
� Is a strong indicator to aid in declaring early in the
season
� Premise� For Kharif
� If area under sowing is less than 33.3% of normal total sown at the
end of July/August due to failure of rainfall or due to late onset of
monsoon – then conditions are potent for drought
� If area under sowing is 50% or less of normal total area sown at
the end of July/August – then “Severe” drought conditions
� For rabi (esp for NE monsoon regions)
� Same criteria at the end of October/November
Decrease in total area
sown as percent of total
normal area sown
Drought Category
33.3 – 50 % Moderate
>= 50% Severe
SOIL MOISTURE BASED INDICES
�Very relevant for agricultural impacts
�Rainfed agriculture region
�Soil moisture is highly dynamic in space
and time
�Network of observatories recording soil
moisture regularly and rigorously is
rudimentary
�Lack of large area representation
�States of invest in setting-up soil moisture
monitoring station and automatic rain
gauges (ARG) through DMC
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PASM: PERCENT AVAILABLE SOIL MOISTURE
� SMw : weekly soil moisture (vol/vol)
� FC: Field capacity of soil vol/vol
� PWP: Permanent wilting point of soil (vol/vol)
� Frequency: weekly
� Initiation: with the dates of actual sowing
� Averaging: over dominant crop growth stages, viz., early-
growth, vegetative growth, reproductive stage
The ranges and class may be suitable modified by the State depending upon the
local dominant crop types, soil properties, etc. in consultation with experts.
PASM: INPUTS NEEDED
� Soil Moisture:
� Direct measurements from observatories
� Calculated from large area simple soil water balance
� Soil Properties:� FC: Field capacity of soil vol/vol
� PWP: Permanent wilting point of soil (vol/vol)
� Soil depth
� Crop:
� Dominant crops
� Crop Phenology: timing of occurrence of growth stages
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DIRECT MEASUREMENT OF SOIL MOISTURE
� Sensor based
� Time/Frequency domain
reflectometry sensor
� Integrated as part of Automatic
Weather Station
� Available in IMD’s AWS (but at
one depth only)
� AWS of SAU’s and ICAR
�Satellite based� Under R&D
� Only top soil SM
� For large grids
SOIL WATER CONSTANTS
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RANGES OF FC, PWP AND AWC
SOIL & CROP DATA SOURCES
�Sources:� State Department of Agriculture
� NBSS&LUP maps
� SLUSI maps
� Pedo transfer functions (SAUs, ICAR)
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SIMPLE SOIL WATER BALANCE APPROACH
� “Leaky Bucket model”
� Also called the inflow-outflow method, is suitable for
large areas (watersheds) over long periods.
� Water Balance: at weekly interval
AET = (I+P+CR)-(RO+D+∆SW)ET is evapotranspiration;
I is irrigation;
P is precipitation;
CR is capillary rise;
RO is run off;
D is deep drainage and
∆SW is change in soil water content (SWt+1 – SWt)
� The AET is estimated from the FAO Approach
� CR can normally be assumed to be zero when the water
table is more than about 1 m below the bottom of the root
zone.
� RO is estimated by standard hydrological methods (ASAE
curve number)
SIMPLE SOIL WATER BALANCE APPROACH
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MAI: MOISTURE ADEQUACY INDEX
� AET : weekly actual crop evapotranspiration
� PET / RET: weekly potential / reference evapotranspiration
� PWP: Permanent wilting point of soil (vol/vol)
� Frequency: weekly
� Initiation: with the dates of actual sowing
� Averaging: over dominant crop growth stages, viz., early-
growth, vegetative growth, reproductive stage
� (MAI) is a better measure for assessing the degree of
adequacy of rainfall and soil moisture to meet the potential
water requirement of crops
EVAPOTRANSPIRATION (ET)
� Composed of two subprocesses
� Evaporation occurs on surfaces of open water or from vegetation
and ground surfaces.
� Transpiration is the removal of water from the soil by plant
roots, transported through the plant into the leaves and
evaporated from the leaf’s stomata.
� Typically combined in mass balance equations because
the components are difficult to partition.
Evapotranspiration
EvaporationTranspiration
Open
Water Soil Vegetation Surfaces Plants
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POTENTIAL VS. ACTUAL ET
� Potential ET (PET) is the amount of
evaporation that will occur if an unlimited
amount of water is available.
� Actual ET (AET) is the actual amount of
evaporation that occurs when water is limited.
For large areas can use a soil water balance
approach to calculate.
MEASURING ET
� Direct measuring methods of ET or Water Vap Flux
� Lysimeters
� Eddy Covariance method (EC) balance method (BREB)
� Large aperture scintillometer (LAS)
� Bowen ratio energy balance method (BREB)
� Indirect estimation methods
� Field water balance
� Pan Evaporimeters
� FAO approach
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ET ESTIMATION BY FAO APPROACH: CLIMATIC
� Allen et al. (1998) FAO Irrigation & Drainage Paper No. 56
� FAO approach is also well known as the “Two steps method”
� Useful for single crops and when “reference” conditions are
available (i.e., no crop water stress)
� Approach can also be applied under non-standard conditions (i.e.
crop under water stress or other stresses)
� Tries to separately account for different factors affecting Crop ET
� the meteorological factors: net radiation, temperature, wind Speed, vapor
pressure deficit
� the crop factors: canopy architecture, growth stage, stomatal resistance,
stomtal density, cuticular layer, LAI, rooting depth, wind
� the stress factors: (soil moisture stress) soil water holding capacity, depth,
initial water content, texture/structure etc.
REFERENCE EVAPOTRANSPIRATION (RET): FAO� ET rate from a reference surface, not short of water.
� Reference surface: "A hypothetical reference crop with an assumed
crop height of 0.12 m, a fixed surface resistance of 70 s m-1 and an
albedo of 0.23.“
� ET0 is water lost by evaporation from soil surface and through transpiration
during a period from an extensive surface of green grass of uniform height,
actively growing, completely shading the ground and not short of water.
� Only factors affecting ET0 are climatic parameters
� Can be computed from weather data
� Penman-Monteith combination eq.:
� where, ET0 is reference evapotranspiration [mm day-1];
� Rn is net radiation at the crop surface [MJ m-2 day-1];
� G is soil heat flux density [MJ m-2 day-1];
� T is mean daily air temperature at 2 m height [°C];
� u2 is wind speed at 2 m height [m s-1];
� es is saturation vapor pressure [kPa];
� ea is actual vapor pressure [kPa];
� es - ea is saturation vapor pressure deficit [kPa];
� ∆ is slope vapor pressure curve [kPa °C-1]; γ is psychrometric constant [kPa °C-1]
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POTENTIAL EVAPOTRANSPIRATION (PET): THORNTHWAITE
� Needs only actual temperature observations
EVAPOTRANSPIRATION UNDER STANDARD CONDITIONS (ETC)
� Is the evapotranspiration from disease-free, well-fertilized crops, grown in
large fields, under optimum soil water conditions, and achieving full
production under the given climatic conditions.
� Also called “Crop water requirement”
ETc = Kc . RET
� Crop Coefficient (Kc)
� Kc represents an integration of the effects of four primary characteristics
that distinguish the crop from reference grass.
� (i) crop height (ii) albedo (reflectance) of the crop-soil surface, (iii) canopy
resistance, and (iv) evaporation from soil, especially exposed soil
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KC TABLES
EVAPOTRANSPIRATION UNDER NON STANDARD
CONDITIONS (AET)
� Is the evapotranspiration from crops grown under management and
environmental conditions that differ from the standard.
� is calculated by using a water stress coefficient Ks and/or by adjusting Kc for
all kinds of other stresses
AET = Ks . ETc
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PAN EVAPORATION� Oldest / simplest method to
measure evaporation
� Measure water depths in a pan
� U.S. Weather Bureau has standard Class A pan� Cylindrical container made of
galvanized steel
� 10 inches deep and 48 inches in diameter
� Pan placed on a 6 inch wooden platform
� Site should be flat and free of obstructions
� Water filled to 8 inches deep� Refill when water drops to 7
inches deep
� Water level measurements made using a hook gage� Measurements to 0.01 inch
� PET=kp. Epan
Monthly Kp for IARI Station
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CASE STUDY� Crop: Soybean
� DOS: 17th July in 2010
� Variety: Pusa-9712 and Pusa-9814
� Soil Profile depth: 1000 mm
� Soil FC: 25% Soil PWP: 10%
� Initial Soil Moisture: 70% of Available Water Capacity
INPUT PARAMETERS
� Initial soil moisture
� ET0 by P-M method (Allen et al., 1998)
� Phenology for the assignment of Kc value
� Soil stress coefficient (Ks)
� FC, PWP, MAD (0.5)
� Irrigation
� Rainfall
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OUTPUT PARAMETERS
�Daily AET
�Daily remaining soil
moisture in the profile
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DAILY VARIATION OF AET
Daily actual evapotranspiration of JS 335 (2009)
0
1
2
3
4
5
6
7
170 190 210 230 250 270 290 310
Julian day
Actu
al
Evap
otr
an
sp
irati
on
(mm
)
COMPARISON OF OBSERVED AND
SIMULATED SOIL MOISTURE
Observed and simulated soil moisture storage (JS 335)
100.0
120.0
140.0
160.0
180.0
200.0
220.0
240.0
180 200 220 240 260 280 300
Julian day
so
il m
ois
ture
sto
rag
e (
mm
)
Simulated soil moisture storage Observed soil moisture storage
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http://www.fao.org/nr/water/infores_databases_cropwat.html
MAI CALCULATION FOR AP
Drought severity
MAI
No drought MAI > 0.75
Mild drought MAI <0.75 and >0.50
Moderate
drought
MAI <0.50 and >0.25
Severe drought MAI <0.25
Table 2 Drought classification based on MAI
•Thornthwaite and Mather (1955) weekly water balance model
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RESOURCE PERSONS
� Agrometeorologist
� Agricultural Physicist
� Agricultural Engineer
� Agronomy
� State Agricultural Universities:
� Soil Science
� Agrometeorology
� Agronomy
� ICAR Institutes
� State Remote Sensing Centers
� State Department of agriculture
� Irrigation engineers
Thank You