Crop Water Requirement and Duty Questions and Answers

Answer: A

Duty D (ha/cumec), base period B (days) and delta Δ (m) are related by \(\Delta = 8.64\,B/D\). The constant 8.64 comes from converting 1 cumec flowing for one day (86,400 m\(^3\)) into hectare-metres. Therefore

The values 0.423, 0.542 and 0.611 m result from using the wrong base period or the wrong constant in the relation, for example forgetting that the duty is expressed for the whole base period of 60 days.

Answer: C

The Kor depth is the depth of water to be supplied during the Kor period, the first and most critical watering of a crop. The volume needed is the area times the depth: 2200 ha = 22 x 10\(^6\) m\(^2\), so the volume is \(22 \times 10^6 \times 0.14 = 3.08 \times 10^6\) m\(^3\). The Kor period of 20 days is 1.728 x 10\(^6\) s, so the discharge is

The options 1.56, 1.63 and 1.91 m\(^3\)/s would come from taking the depth or the period slightly wrong; they do not correspond to the data given.

Answer: A

Duty on the field is the area irrigated by one cumec over the base period, and for a delta Δ and base period B it is \(D = 8.64\,B/\Delta\). Here B = 12 days and Δ = 0.40 m, so

Since 20 percent of the water is lost in the water course, only 80 percent of the water at the head reaches the field, so the duty at the head of the water course is lower: \(259.2 \times 0.8 = 207.36\) ha/cumec. The value 259.2 is the duty at the field without the loss. The area of 6 km\(^2\) is not needed for the duty.

Answer: C

The net depth that the canal has to supply is the crop requirement less the effective rainfall: 60 - 20 = 40 cm = 0.40 m over a base period of 20 days. The duty at the field for this crop is

With 20 percent losses on the way, only 80 percent of the water at the head reaches the field, so the duty at the head is \(432 \times 0.8 = 345.6\) ha/cumec. The intensity of irrigation and the command area are not needed for the duty, which is stated per cumec on the crop area.

Answer: A

The Christiansen Uniformity Coefficient is 93.26%.

Christiansen's coefficient is

\[ CU = 100\left(1 - \frac{\sum|x_i - m|}{n\,m}\right) \]

where m is the mean depth. The 12 depths run evenly from 50 to 39 mm, so \(m = (50 + 39)/2 = 44.5\) mm. The absolute deviations are 5.5, 4.5, 3.5, 2.5, 1.5, 0.5 on each side, totalling \(2 \times 18 = 36\) mm:

\[ \begin{aligned} CU &= 100\left(1 - \frac{36}{12 \times 44.5}\right) \\ &= 100(1 - 0.0674) = 93.26\% \end{aligned} \]

91.66 and 89.70 do not follow from Christiansen's formula with these depths, and 36.28 is far too low for depths that vary by only about ±12% around the mean.

Answer: B

It is called the crop period.

The crop period is the whole time from sowing to harvesting. The base period is a related but shorter quantity: the time between the first watering for sowing and the last watering before harvest. So the crop period is slightly longer than the base period, and the two are often taken as equal in rough design.

The rotation period is the interval between two successive waterings of the same field. 'Growth period' is not the term used in irrigation engineering.

Answer: B

The correct sequence is GIR > FIR > NIR > CIR.

Each requirement adds a loss to the one before it:

• CIR, the consumptive irrigation requirement, is the consumptive use less the effective rainfall.
• NIR, the net irrigation requirement, adds water for leaching and pre-sowing to the CIR.
• FIR, the field irrigation requirement, is NIR divided by the field application efficiency.
• GIR, the gross irrigation requirement, is FIR divided by the conveyance efficiency of the canals.

So GIR is the largest and CIR the smallest. Option A reverses the first comparisons, and the other two put CIR or FIR out of place.

Answer: B

The equation is WR= IR + ER + S.

The water requirement of a crop can be met from three sources: water applied by irrigation (IR, measured at the field head), the part of rainfall that the crop can use (effective rainfall, ER), and the moisture already stored in the root zone that the crop draws on (S). The total requirement is their sum.

The options with subtraction rearrange the same terms wrongly. IR alone equals WR minus ER minus S, so writing WR as IR minus any of the other sources reverses the relation.

Answer: A

It is Irrigation Scheduling.

Irrigation scheduling decides when to irrigate and how much water to apply. The frequency of irrigation depends on the crop's water need at each growth stage and on the soil's capacity to hold water, so deciding the frequency on these grounds is the essence of scheduling.

Water budgeting is the accounting of water inflows and outflows for an area. Irrigation requirement is the quantity of water needed, not its timing. Runoff frequency concerns surface flow, not water applied to crops.

Answer: B

The term is Critical period.

The critical period (critical stage) of a crop for water is the growth stage at which a shortage of soil moisture reduces yield most severely and cannot be made up by later irrigation, such as crown root initiation in wheat or flowering in many crops. Irrigation is scheduled first at these stages when water is limited.

Evapotranspiration is the total water lost by the crop and soil. Delta is the total depth of water a crop needs over its base period, and duty is the area a unit flow of water can irrigate. None names the sensitive stage.