Soil Chemistry

• Electrical Resistance Blocks (Gypsum Blocks): Measure soil moisture tension by calibrating electrical resistance across embedded concentric wire electrodes inside a porous calcium sulfate block against soil water potential.
• Operational Range: Gypsum blocks function effectively in intermediate to dry soils ranging from -0.5 to -15 bars (-0.5 to -50 bars).
• Limitation: In very wet soils (0 to -0.3 bars), gypsum blocks are insensitive because the pore water within the block remains saturated, whereas tensiometers operate best in the wet range (0 to -0.85 bars). (Correct answer: Option 3)

• Formula for Field Water-Use Efficiency (WUE):

$$\text{Field WUE} = \frac{\text{Crop Yield (kg/ha)}}{\text{Total Water Used (ha-mm)}}$$

• Given Values:
• Grain Yield = $4.5\text{ t/ha} = 4.5 \times 1000 = 4,500\text{ kg/ha}$
• Water Requirement = $45\text{ cm} = 45 \times 10 = 450\text{ mm}$
• Calculation:

$$\text{Field WUE} = \frac{4,500\text{ kg/ha}}{450\text{ mm}} = 10\text{ kg/ha-mm}$$

• Conclusion: The field water-use efficiency of the wheat crop is 10 kg/ha-mm. (Correct answer: Option 2)

• Water Resources Potential of India:
• Total annual precipitation: ~4,000 BCM (Billion Cubic Metres)
• Average annual water availability: ~1,869 BCM
• Total utilizable water resources: 1,123 BCM
• Breakdown of Utilizable Water:
• Utilizable Surface Water Resources = 690 BCM
• Replenishable / Utilizable Ground Water Resources = 433 BCM
• Conclusion: The estimated total utilizable surface water resources of India is 690 BCM. (Correct answer: Option 2)

• Statement (I) is true: A mass curve of rainfall is a cumulative plot of rainfall depth against time ($P$ vs $t$). The instantaneous slope at any point along the curve represents the rate of rainfall depth per unit time ($\frac{dP}{dt}$), which directly defines the rainfall intensity at that interval.
• Statement (II) is true: A hyetograph is a bar graph of rainfall intensity versus time intervals, which is mathematically derived directly from the step-wise slopes of the cumulative mass curve.
• Conclusion: Both Statement (I) and Statement (II) are true. (Correct answer: Option 1)