Soil Water Relationship

• The water below the water table is known as ground water and above the water table is known as soil – moisture or soil – water.
•  Soil provides the necessary medium the plants to the water through which water gets used by the plants.
•  Soil moisture can be classified as following.

1)      Gravity water

2)      Capillary water

3)      Hygroscopic water

4)      Structural water

1)      Gravity water – Gravitational water is that water which is not held by the soil as this water drains out freely under the action of gravity. गुरुत्वाकर्षण पानी वह पानी है जो मिट्टी द्वारा धारण नहीं किया जाता है क्योंकि यह पानी गुरुत्वाकर्षण की क्रिया के तहत स्वतंत्र रूप से बाहर निकलता है।

2)      Capillary water –

• Capillary water is that water which is retained in the soil.
  केशिका पानी वह पानी है जो मिट्टी में बरकरार रहता है।
• Water is held in the soil by force of surface tension against the force of gravity.
  गुरुत्वाकर्षण बल के विरुद्ध सतह के बल द्वारा मिट्टी में पानी का रुक जाना।
• This water is most beneficial for plant growth.
• It is also known as available water.

3)      Hygroscopic water –

• Hygroscopic water is that water which is absorbed by the particles of dry soil and is held as a very thin film on the surface of particles due to adhesion.
  हाइग्रोस्कोपिक पानी वह पानी है जो सूखी मिट्टी के कणों द्वारा अवशोषित किया जाता है और आसंजन के कारण कणों की सतह पर बहुत पतली फिल्म के रूप में आयोजित किया जाता है।
• It can not removed easily from the soil particles.
• It is not available for plant growth.
• It can be removed only at very high temperature.

4)      Structural water –

• It is that water which is present in the chemical bonds between the molecules.
  यह वह पानी है जो अणुओं के बीच रासायनिक बंध में मौजूद होता है।
• It is not removed but if removed it changes the soil properties.

Basic Definitions –

1. Field Capacity ( F.C ) –

• It is the amount of water content held in the soil after excess water gets drained off due to gravity. यह गुरुत्वाकर्षण के कारण अतिरिक्त पानी के बह जाने के बाद मिट्टी में बची जल की मात्रा है।
• It is expressed the ratio of the weight of water contained in the soil to the weight of dry soil retaining that water.
• In medium textured soils, field capacity is about 50% of pore volume.

      II  Saturation capacity -

• It may be defined as the total water content of soil when 100% of pores are filled with water.
  100% छिद्र पानी से भर जाने पर इसे मिट्टी की कुल जल सामग्री के रूप में परिभाषित किया जा सकता है।
• It is the maximum water holding capacity of soil.

    III  Permanent wilting point/ स्थायी विल्टिंग पॉइंट –

• Water content at which plant is no longer able to extract water from the soil for its growth.
• At PW Pont, films of water are held very tightly and hence plant roots not able to extract sufficient water.

     IV  Available moisture (AM) –

•  AM = F.C – P.W.P

     V    Readily Available moisture (RAM) –

• It is the portion of available moisture which can be most easily extracted by the plants. यह उपलब्ध नमी का हिस्सा है जो पौधों द्वारा सबसे आसानी से निकाला जा सकता है।
• In general, RAM is 75% of the available moisture.

Depth of water stored in root zone of soil (d_w)

Let d be the depth of root zone , dw be the depth of water stored in root zone & Fc be the field capacity.

If we consider 1 m2 area of soil of depth of root zone d.

Volume of soil = d x 1 cubic meter

If the dry unit wt. of soil is ɣ_d  & ɣ_w is unit wt of water

                      Wt. of water retained in soil = ɣ_w x d_w  x 1

Wt. of dry soil having same volume = ɣ_d x d x 1

                         Field capacity (F.C) = (ɣ_w x d_w)/(ɣ_d x d)

                                                    d_w = (ɣ_d x d) x F.C/ ɣ_w

                                                   d_w  = (ɣ_d x d) x (F.C – P.W.P)/ ɣ_w

Irrigation requirements

Consumptive use/ Epavapotranspiration (Cu) –

                 Consumptive use for a particular crop is defined as the total amount of water used by plants in transpiration and evaporation from adjacent soils, in any specified time.

                किसी विशेष फसल के लिए उपभोग का उपयोग किसी भी निर्दिष्ट समय में, निकटवर्ती मिट्टी से वाष्पोत्सर्जन और वाष्पीकरण में पौधों द्वारा उपयोग किए जाने वाले पानी की कुल मात्रा के रूप में परिभाषित किया गया है।

(1)    Consumptive irrigation requirement (C.I.R) - 

• It is water required through irrigation for consumptive use.
  यह जल के माध्यम से सिंचाई के लिए आवश्यक है।
• C.I.R =  Consumptive water requirement – Effective rainfall

(2)          Net Irrigation Requirement ( N.I.R) –

• It is the amount of irrigation water required in the field to meet the needs of consumptive use as well as other requirement such as leaching.
  यह खेत में सिंचाई के लिए आवश्यक पानी की मात्रा का उपयोग करने के साथ-साथ अन्य उपयोग जैसे कि लीचिंग के लिए आवश्यक है।
• N.I.R = C.I.R + Other req/ Leaching Req

(3)          Field Irrigation Requirement (F.I.R) –

• It accounts for losses occurring in the field.
• F.I.R = N.I.R/ Application Efficiency

(4)          Gross Irrigation Requirements (G.I.R) –

• It is the total quantity of water which is required by taking into account conveyance losses.
• G.I.R = F.I.R/ Conveyance Efficiency

Important questions

Q01 Wheat is to be grown in field having field capacity 27% and permanent wilting point is 13%. Find the storage capacity in 80 cm depth of soil, if the dry unit wt of the soil is 1.5 g/cc. If the irrigation water is to be supplied when the average soil moisture falls to 18%, find the water depth required to be supplied to field.

Solution – Given values

                F.C = 27%, P.W.P = 13% , R.A.M = 18% , ɣ_d = 1.5g/cc

                (a) Maximum storage capacity = d_w  = (ɣ_d x d) x (F.C – P.W.P)/ ɣ_w

                                                                                                           d_w  =  (1.5 x 80) x ( 0.27 – 0.13)/1

                                                                       d_w  = 16.8 cm

                (b) Depth of irrigation water required to be supplied to the field

                                                                      d_w  = (ɣ_d x d) x (F.C – R.A.M)/ ɣ_w

                                                                                                        d_w  = (1.5 x 80) x ( 0.27 – 0.18)/1

                                                                      d_w  = 10.8 cm

Q02 Depth of water in root zone at filed capacity and permanent wilting point are 0.5 m & 0.2 m per meter depth of soil respectively. Find the F.C & P.W.P. Take ɣ_d = 13.73 kN/cubic meter.

Solution – Given values

                d = 1 m , d_w1 (at F.C)= 0.5 m , d_w2 (at P.W.P)= 0.2 m

(a)     F.C = Wt. of water retained in root zone corresponding to F.C/ Wt. of dry soil

                        = ɣ_w  x (d_w1 x 1)/ ɣ_d x ( d x 1)

                                 = 9.81 x (0.5 x 1) / 13.73( 1 x1 )

                                 = 0.3573 = 37.52%

(b)  P.W.P = Wt. of water retained in root zone corresponding to P.W.P/ Wt. of dry soil

                        = ɣ_w  x (d_w2 x 1)/ ɣ_d x ( d x 1)

                                 = 9.81 x (0.2 x 1) / 13.73( 1 x1 )

                                = 0.1429 = 14.29%