(i) the surface of the Salton Sea lies below sea level, «the reference point (datum) used for the vertical axis» ✔

(ii) the rate at which the water level falls increases between 1970 and 2000, «the graph becomes steeper (more negative gradient) over this period» ✔

supported by gradient values, e.g. gradient near 1970 (using 1960 and 1980) $\approx -0.045\ \mathrm{m\,yr^{-1}}$, gradient near 2000 (using 1990 and 2010) $\approx -0.27\ \mathrm{m\,yr^{-1}}$, «about six times as steep» ✔

(iii) «height fallen $= |-73.6-(-71.5)| =$» 2.1 «m» ✔

«$V = 900\times 10^{6}\ \mathrm{m^2} \times 2.1\ \mathrm{m} =$» $1.89\times 10^{9}\ \mathrm{m^3}$

(iv) Any two of:

reduced inflow of water into the lake «because water that used to drain into it is now diverted for irrigation or urban use» ✔

OR

increased evaporation caused by higher air temperatures «consistent with climate change» ✔

OR

little or no precipitation in the surrounding desert catchment «prolonged drought» ✔

(i) [1] for linking the negative values to the water level being below the datum/reference point (sea level); an answer that only restates ‘the lake is losing water’ without reference to the datum earns nothing. (ii) M1: correct qualitative trend (the rate of fall increases / the graph becomes steeper); M2: quantitative gradient evidence read from the graph, accept any pair of values that clearly shows the gradient near 2000 is larger in magnitude than the gradient near 1970. (iii) M1: correct height change, 2.1 m (accept 2.0-2.2 m from reading the graph); M2: correct final volume with the correct unit, ECF from M1. (iv) [1] each for two distinct, plausible environmental causes; two different phrasings of the same underlying idea (e.g. ‘less rain’ and ‘drought’) score once only.