the rootzone of alfalfa is 1.8 meter deep.the soil in this rootzone contain 5 inch water.How many liters of soil moisture does this soil contain per cubic meter?
Answers
Answer:
Problems and Solutions to Chapter 14: Soil-Water Relations & Evapotranspiration
Soil Water Relations is worth 15 points (5 points per problem, Problem 14.2 is extra credit).
Problem14.1
Determine the available water in loam soil for a crop with a 3-ft root zone. What is the readily available water if wheat is being grown?
Solution to 14.1
a. AW = [FC(v) - PWP(v)]Dr/100
where:
AW = depth of water available, ft or in. = 2.0 in./ft (from Table 14.1)
FC(v) = volumetric field capacity = 31% (from Table 14.1)
PWP(v) = volumetric permanent wilting point = 14% (from table 14.1)
D(r) = depth of root zone = 3 ft(from Table 14.2)
AW = (31-14)3 ft/100 = 0.51 ft or approx. 6 in.
OR
From Table 14.1, loam soil, total available water = 2.0 in./ft
AW = (2.0 in./ft)(3 ft) = 6 inches
b. RAW = (MAD)(AW)
where:
MAD = management allowed defiency
AW = depth of water available, ft or in.
From Table 14.1, AW = 2.0 in./ft
From Table 14.2, rooting depth wheat is approx. 4 ft (3-5 ft), MAD (wheat) = 0.55
RAW = (0.55)(2.0 in./ft)(4 ft) = 4.4 inches (3.3 in. - 5.5 in.)
Problem 14.2
Estimate how much water has been depleted from 3 ft of a loam soil if the dry-weight water content is 15 percent?
Solution for 14.2
P(v) = P(d) * A(s)
where:
P(v) = volumetric water content = wet weight basis
P(d) = dry-weight water content
A(s) = soil apparent specific gravity
P(v) = (15)(1.4) = 21%
Assume soil was originally at Field Capacity = 31% moisture content
% depleted = (original volumetric moisture content - final volumetric moisture content)/original volumetric moisture content
= (31% - 21%)/31% = 32% depleted
Amount of water @ FC = (31)(3 ft)/100 = 0.93 ft
Amount of water currently = (21)(3 ft) /100 = 0.63 ft
Which means that 0.30 ft of water has been depleted or 3.6 in.
Problem 14.3
Cotton is being grown in July on a field where the top 2 ft of soil is a clay loam and the next 2 ft is a sandy loam. Determine the available water and readily available water. If cotton used 0.3 in./day, how often should irrigation occur if no rainfall occurs?
Solution to 14.3
From Table 14.1, clay loam = 2.3 in./ft, sandy loam = 1.4 in./ft
From Table 14.2, cotton D(r) = 4 ft (3-5 ft), MAD = 0.6
AW(total) = AW(0-2 ft) + AW(2-4 ft) = (2.3 in./ft)(2 ft) + (1.4 in./ft)(2 ft) = 7.4 inches
RAW = (7.4 inches)(0.6) = 4.4 inches
irrigation period = 4.4 in. x day/0.3 in. = 14.7 days or 15 days
Problem 14.4
Corn is being grown on a clay loam soil in the Midwest. Estimate the readily available water, assuming the plants are starting to tassel.
Solution to 14.4
From Table 14.1, TAW = total available water = 2.3 in./ft
From Table 14.2, corn D(r) = 5 ft (3-5 ft), MAD = 0.6
RAW = (MAD)(AW) = (0.6)(2.3 in./ft)(5 ft) = 6.9 inches (4.1 - 6.9 inches)
Evapotranspiration Section is worth 15 points (5 points per problem).
Problem 14.5
Determine the seasonal water requirement for a crop in your area using the Blaney-Criddle method.
Solution to 14.5
U = KF = K f = kf = k(tp)/100
where:
U = seasonal water use, in.
K = seasonal evapotranspiration coefficient
F = sum of monthly evapotranspiration factors f for period
f = monthly evapotranspiration factor
k = monthly evapotranspiration coefficient
t = mean monthly temperature, °F
p = monthly percent of annual daytime hours (Table 14.5)
cotton is grown in Yuma - March to September
cotton is grown in Tucson - April to October
From Table 14.6: K = 0.60 - 0.70, choose 0.70 (for arid climates - see footnote)
Latitudes: Tucson = 32° 16'49" (from Azmet Web site)
Yuma = 32° 42'45"
Month
Temp, °F
Daytime Hours, %
f
Effective Rainfall, in.
T
Y
T
Y
T
Y
T
Y
Mar
62
8.37
5.19
0.41
Apr
61
67
8.76
8.76
5.34
5.87
0.49
0.01
May
72
74
9.62
9.62
6.93
7.12
0
0
Jun
81
82
9.59
9.59
7.77
7.86
0
0
Jul
86
93
9.77
9.77
8.40
9.09
3.37
0.11
Aug
85
94
9.29
9.27
7.88
8.71
1.14
0.08
Sep
82
86
8.34
8.34
6.84
7.17
0.93
1.01
Oct
69
7.95
5.49
0.11
Sum
48.65
51.01
6.04
1.62
U(yuma) = K (sum)tp/100 = (0.70)(51.01 in.) = 35.7 inches
U(tucson) = K (sum)tp/100 = (0.70)(48.65 in.) = 34.06 inches
Problem 14.7
What is the estimated water use for soybeans in eastern Nebraska for June 1, assuming Eto is 0.28 in./day?
Solution to 14.7
ETc = EtoKc
where:
Etc = estimated ET for crop, in./d
Eto = estimated gross reference ET, in./d
Kc = crop coefficient
From Table 14.8, Kc = 0.6 (soybeans, June 1, Midwest)
Etc = (0.28 in./day)(0.60) = 0.17 in./day
Problem 14.9
Determine the alfalfa-based crop coefficient for field corn on June 30 and August 10 if planting is May 1, and effective cover occurs July 10.
Solution to 14.9
May 1 = plant
June 30 = (31 +30) = 61 days
July 10 = effective cover (31 + 30 + 10) = 71 days
August 10 (31 - 10 + 10) = 31 days after effective cover
61/71 = 86% from planting to effective cover
Using Table 14.9 and interpolating:
(90-86)/(0.75-x) = (90-80)/(0.75-0.55)
x = -4(0.20)/10+0.75(10) = 0.67 = Kc (June 30)
August 10:
(40-30)/(0.8-0.9) = (31-30)/(x-0.9)
x = -0.1 + 9.0/10 = 0.89 = Kc (August 10)